feat(keys): 每个密钥可配独立 AUTO 链，管理员代配

此前 AUTO 链是全局单值（cfg.Auto + Core.AutoChain()），所有用户共用一条链。
管理员无法为某个用户单独指定调度链。

按 per-key 覆盖 + 全局兜底实现：

- config.GWKey 增加 Auto 字段与 HasOwnAuto()。未配置即继承全局链，
  存量部署零改动，新密钥天然继承全局链。
- Core 把 buildAutoChain 的归一化逻辑抽成 chainForRules，全局链、
  生图链、per-key 链共用同一套编译，避免两处漂移。
- Core 增加 keyAutoChains 缓存 + AutoChainFor(key)。请求路径读缓存不
  加锁，与全局链查询一致。缓存整表原子替换，不会看到半成品。
- 请求侧 chat.go 改用 AutoChainFor(reqKey)。冷却仍在 Provider 上按
  model+source 共享：两条链指向同一个 slot 时共用冷却，与今天单链行为
  相同，也避免为 per-key 维度重构冷却而改变现有可观测语义。
- API：GET/PUT/DELETE /api/keys/{key}/auto（admin），GET
  /api/keys/me/auto（任意角色，只能读自己的）。空 PUT 与 DELETE 等价于
  "恢复继承"，无法持久化一条会 503 的空链。写入时回报解析出的槽位数，
  让管理员当场看到模型名写错，而不是等用户下次请求 503。
- 源变更时一并重编译 per-key 链，加源后无需重启即可生效。

判据 18 条，5 个变异全部被抓住：AutoChainFor 忽略 key、清空后不重建
缓存、源变更不重建、空 PUT 落盘成空链、me/auto 误要求 admin。

UI 判据做变异时发现漏放：只查函数定义存在，删掉按钮后仍通过。已改为
断言 keyCanvasHtml 内的调用点。

端到端实测（真实 HTTP + 两个 mock 上游）：admin 与 bob 初始同为 m-fast，
给 bob 配 m-cheap 后两者分流，重启后仍分流，DELETE 后 bob 回到 m-fast。

Co-Authored-By: ModelRouter <noreply@modelrouter.dev>

internal/gateway/ui/index.html

@@ -822,6 +822,17 @@
           keysHint:
             "管理员密钥可查看与管理全部密钥，并可为每个用户密钥配置可用模型范围；用户密钥只能看到自己。",
           kCreate: "新建密钥",
+          kAutoChain: "AUTO 链",
+          kAutoChainOwn: "独立链",
+          kAutoChainGlobal: "跟随全局",
+          kAutoChainTitle: "独立 AUTO 链",
+          kAutoChainHint: "为空则跟随全局 AUTO 链",
+          kAutoChainInherit: "使用全局链",
+          kAutoChainAdd: "+ 添加槽位",
+          kAutoChainSave: "保存",
+          kAutoChainTier: "层级",
+          kAutoChainEmpty: "该密钥使用全局 AUTO 链",
+          kAutoChainBad: "槽位无法解析（模型或源不存在），保存后 AUTO 请求会失败",
           kName: "名称",
           kRole: "角色",
           kRoleAdmin: "管理员",
@@ -1081,6 +1092,17 @@
           keysHint:
             "Admin keys can view and manage every key and configure each user key\u0027s allowed models; user keys only see themselves.",
           kCreate: "Create key",
+          kAutoChain: "AUTO chain",
+          kAutoChainOwn: "own",
+          kAutoChainGlobal: "global",
+          kAutoChainTitle: "Per-key AUTO chain",
+          kAutoChainHint: "Leave empty to inherit the global AUTO chain",
+          kAutoChainInherit: "Use global chain",
+          kAutoChainAdd: "+ Add slot",
+          kAutoChainSave: "Save",
+          kAutoChainTier: "Tier",
+          kAutoChainEmpty: "This key uses the global AUTO chain",
+          kAutoChainBad: "Slot does not resolve (unknown model or source); AUTO requests would fail",
           kName: "Name",
           kRole: "Role",
           kRoleAdmin: "Admin",
@@ -4674,6 +4696,8 @@
       <button class="ghost small" onclick="copyText('${escAttr(k.key)}')">${t("kCopy")}</button>
       <span class="grow"></span>
       <span class="muted">${fmtCreated(k.created_at)}</span>
+      <button class="ghost small" title="${escAttr(t("kAutoChainTitle"))}"
+        onclick="openKeyAutoModal('${escAttr(k.key)}','${escAttr(k.name || "")}')">${t("kAutoChain")}</button>
       <button class="ghost small errc" onclick="delKey('${escAttr(k.key)}','${escAttr(k.name || "")}')">${t("kDel")}</button>
       <button class="ghost small errc" title="${escAttr(t("kDel"))}" onclick="delKey('${escAttr(k.key)}','${escAttr(k.name || "")}')">×</button>
     </div>
@@ -5085,6 +5109,108 @@
         document.body.appendChild(wrap);
         $("#kc-name").focus();
       }
+      // Per-key AUTO chain editor. Kept deliberately simple: a plain table of
+      // slots rather than a drag canvas, because an admin usually either
+      // mirrors the global chain or deletes a few tiers, and the canvas layout
+      // for the model scope list would be overkill here.
+      async function openKeyAutoModal(key, name) {
+        const wrap = document.createElement("div");
+        wrap.id = "modal-wrap";
+        wrap.style.cssText =
+          "position:fixed;inset:0;background:rgba(15,22,44,.45);display:flex;align-items:flex-start;justify-content:center;overflow:auto;padding:48px 20px;z-index:50";
+        wrap.innerHTML = `<div class="card" style="width:720px;max-width:100%">
+    <h2>${esc(t("kAutoChainTitle"))} · ${esc(name || key)}</h2>
+    <div class="muted" style="margin-bottom:10px">${esc(t("kAutoChainHint"))}</div>
+    <div id="ka-rows"></div>
+    <p><button class="ghost" onclick="keyAutoAddRow()">${esc(t("kAutoChainAdd"))}</button>
+       <button class="ghost" onclick="keyAutoClear(this)">${esc(t("kAutoChainInherit"))}</button></p>
+    <p><button onclick="keyAutoSave(this,'${escAttr(key)}')">${esc(t("kAutoChainSave"))}</button>
+       <button class="ghost" onclick="this.closest('#modal-wrap').remove()">${esc(t("mCancel"))}</button></p>
+  </div>`;
+        document.body.appendChild(wrap);
+        try {
+          const j = await api("/api/keys/" + encodeURIComponent(key) + "/auto");
+          const rows = $("#ka-rows");
+          if (!j.auto || !j.auto.length) {
+            rows.innerHTML = `<div class="muted" style="padding:8px 0">${esc(
+              t("kAutoChainEmpty")
+            )}</div>`;
+          } else {
+            j.auto.forEach((r) => keyAutoRow(r));
+          }
+        } catch (e) {
+          toast(String(e));
+        }
+      }
+      function keyAutoRow(r) {
+        const el = document.createElement("div");
+        el.className = "ka-row";
+        el.style.cssText =
+          "display:flex;gap:8px;align-items:center;margin-bottom:8px;flex-wrap:wrap";
+        const opts = (sel) =>
+          (allModels || [])
+            .map((p) => {
+              // loadModelPairs builds {key,label}; the /api/status fallback
+              // only guarantees an id, so derive a comb key from whatever is
+              // present rather than emitting an empty option value.
+              const val = p.key || (p.id ? p.id + (p.source ? "|" + p.source : "") : "");
+              const lab = p.label || p.id || val;
+              return `<option value="${escAttr(val)}"${
+                val === sel ? " selected" : ""
+              }>${esc(lab)}</option>`;
+            })
+            .join("");
+        el.innerHTML = `<select class="ka-model" style="flex:2 1 220px;min-width:0">${opts(
+          r ? scopeComb(r) : ""
+        )}</select>
+    <label class="muted" style="flex:0 0 auto">${esc(
+          t("kAutoChainTier")
+        )}<input class="ka-tier" type="number" min="1" value="${
+          (r && r.tier) || 1
+        }" style="width:64px;margin-left:6px"></label>
+    <button class="ghost small errc" onclick="this.closest('.ka-row').remove()">×</button>`;
+        $("#ka-rows").appendChild(el);
+      }
+      function keyAutoAddRow() {
+        keyAutoRow(null);
+      }
+      function keyAutoClear() {
+        $("#ka-rows").innerHTML = "";
+      }
+      async function keyAutoSave(btn, key) {
+        const rules = [];
+        document.querySelectorAll("#ka-rows .ka-row").forEach((row) => {
+          const comb = row.querySelector(".ka-model").value;
+          if (!comb) return;
+          const { model, source } = splitCombKey(comb);
+          const tier = parseInt(row.querySelector(".ka-tier").value, 10) || 1;
+          rules.push({ model, source, tier });
+        });
+        btn.disabled = true;
+        try {
+          // An empty rule set means "inherit global" server-side, so one PUT
+          // covers both clearing the override and saving a chain.
+          const j = await api("/api/keys/" + encodeURIComponent(key) + "/auto", {
+            method: "PUT",
+            headers: { "Content-Type": "application/json" },
+            body: JSON.stringify({ auto: rules }),
+          });
+          if (j.inherits) {
+            toast(t("kAutoChainGlobal"));
+          } else if (!j.slots) {
+            // Persisted, but nothing resolves — say so instead of letting the
+            // user discover it as a 503 on their next AUTO request.
+            toast(t("kAutoChainBad"));
+          } else {
+            toast(t("kAutoChainSave") + " (" + j.slots + ")");
+          }
+          document.getElementById("modal-wrap").remove();
+          loadKeys();
+        } catch (e) {
+          toast(String(e));
+          btn.disabled = false;
+        }
+      }
       async function createKey(btn) {
         const name = $("#kc-name").value.trim();
         if (!name) {