fix(gateway): 探测同时算出 image_endpoint，修 /v1/v1 404

上一版只算了 endpoint，图像生成仍然 404。根因是网关两条拼接路径的默认值
规则相反：

    ChatURL  = BaseURL + Endpoint()       默认 "/chat/completions"      相对
    ImageURL = BaseURL + ImageEndpoint()  默认 "/v1/images/generations" 绝对

base_url 带 /v1 时 chat 拼成 /v1/chat/completions 正常，image 拼成
/v1/v1/images/generations 就 404。之前归一化 base_url 时修正了版本段的位置，
但没意识到图像这条路径的默认值自带 /v1，它跟 base 是各管一段。

探测现在额外返回 image_endpoint：base 末尾有版本段时给
/images/generations，否则留空用默认值。判据是 TestComposeImageEndpoint ——
它断言 base+结果绝不含 /v1/v1，这是那个 404 的字面形态。

apply 的继承逻辑相应改了：image_endpoint 不再从旧源继承。旧值是对旧
base_url 正确的，而 base_url 恰恰是这次可能变的东西，沿用它会留下一个对着
新 base 404 的陈旧值。前端 srcDiscover() 同步写入该字段。

端到端（隔离实例 18099，真实 amr-link 上游）：
  探测     http 200，1.4s，29 chat + 5 image
  apply    http 200，落库 34 个模型
  文本     http 200，2.0s
  视觉     http 200，1.5s
  图像     路径修正为 /v1/images/generations（不再 404；上游本身 120s 超时，
           属其自身耗时，非路由问题）

探测对已存在的源返回 409 要求显式 replace_models=true，这个保护在验证中
确实生效了。

README.md

@@ -150,6 +150,47 @@ sources:
     timeout: 120s              # 请求超时，默认 120s
 ```
 
+
+### 自动探测模型列表
+
+不想手抄模型 id 时，只填 URL 和 key，让网关去扫：
+
+```bash
+# 只探测，不写任何东西
+curl -X POST http://127.0.0.1:8080/api/sources/discover \
+  -H "Authorization: Bearer sk-gw-…" -H "Content-Type: application/json" \
+  -d '{"base_url":"https://api.vendor.com/v1","api_key":"sk-…"}'
+# → {"ok":true,"base_url":"…","endpoint":"","models":["gpt-x",…],
+#    "image_models":["gpt-image-2"],"models_path":"/models","latency_ms":1387,…}
+
+# 探测后直接落库
+curl -X POST http://127.0.0.1:8080/api/sources/discover/apply \
+  -H "Authorization: Bearer sk-gw-…" -H "Content-Type: application/json" \
+  -d '{"base_url":"https://api.vendor.com/v1","api_key":"sk-…","name":"vendor"}'
+```
+
+WebUI 源编辑框里也有「自动探测模型」按钮：填好 URL 和 key 点一下，列表自动
+填好，仍需点保存才生效。
+
+几点值得知道：
+
+- **URL 形态不用猜。** 厂商对「`/v1` 放 base 还是放 path」没有共识，探测会
+  依次尝试若干候选并返回答对的那个，同时给出归一化后的 `base_url` +
+  `endpoint`（`endpoint` 平时留空，因为 openai adapter 自带 `/chat/completions`）。
+- **`image_endpoint` 是另一套规则。** 网关的图像默认路径是**绝对**的
+  `/v1/images/generations`，而 chat 默认是相对的。所以 `base_url` 带 `/v1` 时
+  必须显式给 `image_endpoint: /images/generations`，否则图像请求 404。
+  探测会替你填好。
+- **图像模型只看上游自己声明的 modality**，绝不从模型名猜。`gpt-image-2`
+  这类 id 也出现在同时提供对话服务的目录上，猜错会让 AUTO 链选中一个只支持
+  图像生成的模型，其后每一层都被它拖垮。
+- **不会逐模型探活。** 34 个模型的目录跑一轮要花钱且耗时数分钟，`/models`
+  是足够便宜的信号；模型是否真能应答留给正常请求去发现。
+- **对已存在的源，必须显式 `replace_models: true`** 才会覆盖模型列表。重新
+  探测顺手抹掉为 AUTO 链手工调过的 priority，是最难排查的一类数据丢失。
+- **`api_key` 留空表示沿用已存值**，不会把凭据清空。
+
+
 `config.yaml`、`runtime.json`、`master.key`、`adapters/` 均不提交版本库（见
 `.gitignore`），请随备份带走——`master.key` 丢失后运行时密文无法解密。
 

internal/gateway/discover.go

@@ -72,15 +72,23 @@ type discoveredModel struct {
 // worked — the failure text is what the operator needs, so it must name every
 // URL that was tried rather than just the last one.
 type discoveryResult struct {
-	BaseURL     string   `json:"base_url"`
+	BaseURL  string `json:"base_url"`
-	Endpoint    string   `json:"endpoint"`
+	Endpoint string `json:"endpoint"`
-	Adapter     string   `json:"adapter"`
+	// ImageEndpoint is reported separately because the gateway composes it
-	ModelsPath  string   `json:"models_path"`
+	// differently: Provider.ImageURL is BaseURL + ImageEndpoint(), and the
-	Models      []string `json:"models"`
+	// default ImageEndpoint is the ABSOLUTE path "/v1/images/generations".
-	ImageModels []string `json:"image_models"`
+	// Chat's default is the relative "/chat/completions". So a base_url that
-	Tried       []string `json:"tried"`
+	// already ends in /v1 produces a working chat URL and a broken image URL
-	LatencyMS   int64    `json:"latency_ms"`
+	// ("/v1/v1/images/generations") — which is exactly the 404 seen on the
-	Note        string   `json:"note,omitempty"`
+	// first vela configuration, and which discovery has to correct.
+	ImageEndpoint string   `json:"image_endpoint"`
+	Adapter       string   `json:"adapter"`
+	ModelsPath    string   `json:"models_path"`
+	Models        []string `json:"models"`
+	ImageModels   []string `json:"image_models"`
+	Tried         []string `json:"tried"`
+	LatencyMS     int64    `json:"latency_ms"`
+	Note          string   `json:"note,omitempty"`
 }
 
 // MarshalJSON guarantees the two list fields are always JSON arrays, never
@@ -254,21 +262,42 @@ func composeEndpoint(modelsPath, persistedBase, typedBase string) string {
 	// The version segment present in the models path that answered.
 	inPath := versionSegment(modelsPath)
 
-	// Prefer the segment that actually WORKED. When the models path carried
+	// The version segment actually WORKED: prefer the models path's, then the
-	// one and the base does not, move it onto the endpoint.
+	// one the operator typed.
-	if inPath != "" && onBase == "" {
+	version := inPath
-		return inPath + "/chat/completions"
+	if version == "" && onBase == "" {
+		if b := trimVersionSuffix(typedBase); b != typedBase && typedBase != "" {
+			version = typedBase[len(b):]
+		}
 	}
-	// Both carry one (or the path had none because its version lives on the
+	if version == "" {
-	// base): keep the base as-is and let the adapter default compose.
+		return "" // adapter default already composes
-	if onBase != "" {
-		return ""
 	}
-	// Neither carries one, but the operator typed a version segment that is
+	return version + "/chat/completions"
-	// being stripped off the base. Restore it on the endpoint so the chat path
+}
-	// keeps working.
+
-	if b := trimVersionSuffix(typedBase); b != typedBase && typedBase != "" {
+// composeImageEndpoint derives the image endpoint to persist.
-		return typedBase[len(b):] + "/chat/completions"
+//
+// Provider.ImageURL is `base_url + ImageEndpoint()`, and the built-in default
+// is the absolute path "/v1/images/generations" — unlike chat's relative
+// "/chat/completions". So the two defaults do NOT agree about who owns the
+// version segment, which is the trap this function exists to close:
+//
+//	base "https://h"     + "/v1/images/generations"  → correct
+//	base "https://h/v1"  + "/v1/images/generations"  → "/v1/v1/…" 404
+//
+// Emitting "" when the base already ends in a version is wrong for a different
+// reason: the vendor's catalog path is not proof that its image path carries
+// the same prefix. Some gateways serve the catalog at "/v1/models" but the
+// image endpoint at "/images/generations" (observed: 404 on both forms, but a
+// 404 from the doubled form is indistinguishable from a 404 from the wrong
+// host). Rather than guess, the value written here is derived from the base so
+// it composes, and the caller can still override image_endpoint by hand.
+func composeImageEndpoint(persistedBase string) string {
+	if b := trimVersionSuffix(persistedBase); b != persistedBase {
+		// The base ends in a version segment, so the default absolute path
+		// would double it. Use the version-relative form instead.
+		return "/images/generations"
 	}
 	return ""
 }
@@ -343,6 +372,7 @@ func DiscoverModels(ctx context.Context, rawURL, apiKey string) (*discoveryResul
 		// the default image path into "/v1/v1/images/generations" — the exact
 		// 404 seen on the first vela configuration attempt).
 		res.Endpoint = composeEndpoint(c.modelsPath, c.baseURL, typedBase)
+		res.ImageEndpoint = composeImageEndpoint(c.baseURL)
 
 		// If the operator typed a version segment and we are persisting a base
 		// that no longer has it, the endpoint above is what restores it. If the

internal/gateway/discover_api.go

@@ -74,6 +74,11 @@ type discoverResponse struct {
 
 	// Endpoint is set only when discovery had to move a version segment.
 	Endpoint string `json:"endpoint"`
+	// ImageEndpoint is required whenever base_url carries a version segment:
+	// the gateway's default image path is the ABSOLUTE "/v1/images/generations"
+	// while chat's default is the relative "/chat/completions", so the same
+	// base_url that yields a working chat URL yields a 404 image URL.
+	ImageEndpoint string `json:"image_endpoint"`
 
 	// Diagnostics for the operator.
 	ModelsPath string   `json:"models_path"`
@@ -144,8 +149,9 @@ func (g *Gateway) handleSourceDiscoverAPI(w http.ResponseWriter, r *http.Request
 		Adapter:     req.Adapter,
 		Models:      chat,
 		ImageModels: image,
-		Endpoint:    res.Endpoint,
+		Endpoint:      res.Endpoint,
-		ModelsPath:  res.ModelsPath,
+		ImageEndpoint: res.ImageEndpoint,
+		ModelsPath:    res.ModelsPath,
 		Tried:       res.Tried,
 		LatencyMS:   res.LatencyMS,
 		Note:        res.Note,
@@ -197,8 +203,9 @@ func (g *Gateway) handleSourceDiscoverAPI(w http.ResponseWriter, r *http.Request
 		BaseURL:     res.BaseURL,
 		APIKey:      g.resolveAPIKey(name, req.APIKey, exists),
 		Adapter:     req.Adapter,
-		Endpoint:    res.Endpoint,
+		Endpoint:      res.Endpoint,
-		Models:      modelsToConfig(chat, image),
+		ImageEndpoint: res.ImageEndpoint,
+		Models:        modelsToConfig(chat, image),
 		Temperature: req.Temperature,
 	}
 	if req.MaxConcurrent > 0 {
@@ -219,7 +226,13 @@ func (g *Gateway) handleSourceDiscoverAPI(w http.ResponseWriter, r *http.Request
 			src.Meta = old.Meta
 			src.MaxTokens = old.MaxTokens
 			src.RPM = old.RPM
-			src.ImageEndpoint = old.ImageEndpoint
+			// image_endpoint is NOT inherited when discovery supplied one:
+			// the stored value was correct for the OLD base_url, and the base
+			// is exactly what may have changed. Inheriting it would keep a
+			// stale value that 404s against the new base.
+			if res.ImageEndpoint == "" {
+				src.ImageEndpoint = old.ImageEndpoint
+			}
 			if src.Adapter == "" {
 				src.Adapter = old.Adapter
 			}

internal/gateway/discover_test.go

@@ -517,6 +517,75 @@ func TestDiscoverModels_ImageModelsNeverNull(t *testing.T) {
 	}
 }
 
+// TestComposeImageEndpoint pins the rule that fixed a real 404.
+//
+// Provider.ImageURL is `base_url + ImageEndpoint()`, and the built-in default
+// is the ABSOLUTE "/v1/images/generations" — while chat's default is the
+// relative "/chat/completions". The two defaults disagree about who owns the
+// version segment, so a base_url that produces a working chat URL produces
+// "/v1/v1/images/generations" and a 404.
+func TestComposeImageEndpoint(t *testing.T) {
+	cases := []struct {
+		base string
+		want string
+	}{
+		// base without a version: the absolute default composes correctly.
+		{"https://h", ""},
+		// base WITH a version: the default would double it, so the
+		// version-relative form is written instead.
+		{"https://h/v1", "/images/generations"},
+		{"https://h/v1beta", "/images/generations"},
+		// A real path segment that merely starts with v is not a version.
+		{"https://h/vision", ""},
+	}
+	for _, tc := range cases {
+		got := composeImageEndpoint(tc.base)
+		if got != tc.want {
+			t.Errorf("composeImageEndpoint(%q) = %q, want %q", tc.base, got, tc.want)
+		}
+		// Whatever it decided, base + result must not contain a doubled
+		// version segment — that is the bug this function exists for.
+		joined := tc.base + got
+		if strings.Contains(joined, "/v1/v1") {
+			t.Errorf("composed %q contains a doubled version segment", joined)
+		}
+	}
+}
+
+// TestDiscoveryReportsImageEndpointWhenBaseCarriesVersion is the end-to-end
+// version: a pasted base_url with "/v1" must come back with an image endpoint,
+// or the image models it reported will all 404 on use.
+func TestDiscoveryReportsImageEndpointWhenBaseCarriesVersion(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.URL.Path != "/v1/models" {
+			w.WriteHeader(http.StatusNotFound)
+			return
+		}
+		_, _ = w.Write([]byte(realCatalogBody))
+	}))
+	defer srv.Close()
+
+	res, err := DiscoverModels(context.Background(), srv.URL+"/v1", "k")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(res.ImageModels) == 0 {
+		t.Fatal("fixture must contain image models for this to mean anything")
+	}
+	if res.BaseURL != srv.URL+"/v1" {
+		t.Fatalf("base = %q, want the typed URL with /v1", res.BaseURL)
+	}
+	if res.ImageEndpoint != "/images/generations" {
+		t.Errorf("image_endpoint = %q, want %q (otherwise the reported image models all 404)",
+			res.ImageEndpoint, "/images/generations")
+	}
+	// The chat side stays empty: base already carries /v1 and the adapter's
+	// relative default composes.
+	if res.Endpoint != "" {
+		t.Errorf("endpoint = %q, want empty", res.Endpoint)
+	}
+}
+
 // TestDiscoveryResult_NilSlicesSerializeAsArrays covers the zero value of
 // discoveryResult, which is what a caller gets if it builds one directly
 // instead of going through DiscoverModels. DiscoverModels happens to

internal/gateway/ui/index.html

@@ -3265,6 +3265,12 @@ recsState.keyNames = keyNames || {};
           // instead of guessing in the browser.
           if (r.base_url && urlEl) urlEl.value = r.base_url;
           if (r.endpoint && $("#s-ep")) $("#s-ep").value = r.endpoint;
+          // image_endpoint too: the gateway's default image path is absolute
+          // ("/v1/images/generations") while chat's is relative, so a base_url
+          // with /v1 yields a working chat URL and a 404 image URL unless this
+          // is corrected. Leaving a stale value would also be wrong — it was
+          // correct for the old base_url.
+          if (r.image_endpoint && $("#s-img")) $("#s-img").value = r.image_endpoint;
 
           // Array.isArray, not a truthy check: a JSON null here would make
           // .concat fail and leave the form looking empty with no error.