refactor(quota): 配额改为按模型，删除整钥总配额

用户明确要求：配额应当是密钥对应的**每个模型的单独配额**，而非整体配额。

## 语义变更

删除 GWKey.TokenQuota / ReqQuota / Period / Hours（整钥总额）。
ModelScope 新增 ReqQuota —— 请求数配额下沉到每条模型范围。

现在：每条 models[] 各自带 token 配额 + 请求数配额 + 重置周期，
彼此独立。一个模型用满只影响该模型。

★ 为什么不保留整钥总额：它会让「把 A 模型的额度挪给 B」变成一次全局
重分配；按模型独立计费则每个模型各自可控，运维能直接看出哪个模型在吃预算。

## 连带改动

- checkQuota 合并 key 级与 scope 级判定；checkKeyQuotaRetry 整体删除
  （顺带修掉上轮遗留的双重判定：入口不再先判空再重算）
- core：CreateKeyWithQuota / UpdateKeyWithQuota / ApplyQuota 全部删除，
  改由 ValidateScopeQuotas 校验每条 scope 的配额
- admin key：scope 上的配额不强制（admin 的 scope 仍限制模型范围，
  但不强制配额）—— 否则管理员会把自己锁在门外
- /api/v1/keys 不再回显 key 级配额字段（scope 里已含）
- WebUI：删除整钥配额徽标 / 「配额」按钮 / 创建表单的配额组 /
  putScope 的整钥回传；模型砖块与范围编辑器新增「请求数配额」输入，
  徽标显示 `1.0K 77×·1h`（未设配额显示 ∞）

## 判据

- TestOneModelsQuotaDoesNotBlockAnother 是本次核心保证。
  ★ 它第一版是**假判据**：m2 从不消耗，key-wide 计数器与 m1 自己的计数器
  读数恰好相同，退回 key-wide 仍通过。变异测试抓到后改为「先用 m2 花掉
  远超 m1 配额的量，再验证 m1 仍可用」—— 这样两种设计才可区分。
- TestUncappedModelNeverBlocked / TestAdminKeyScopesAreNotEnforced 新增
- UI 契约判据重写：整钥配额界面必须彻底消失（13 个符号）、
  scope 编辑器必须往返 req_quota、putScope 只发 scope 列表
- 错误消息点名具体模型（TestKeyAPIRejectionNamesTheModel）
- 3/3 变异全被抓

实测（真实进程 + 浏览器）：m2 配额 500000 连打 25 次全成功，
m1 配额 1000 立即 429「token quota exceeded for "m1" (4315/1000)」，
此后 m2/m3 仍 200。UI：整钥配额元素全为 0，砖块各显配额，
编辑器预填/保存正确，零 JS 异常。

internal/config/config.go

@@ -375,21 +375,11 @@ type GWKey struct {
 	Note      string       `yaml:"note,omitempty" json:"note,omitempty"`
 	CreatedAt int64        `yaml:"created_at,omitempty" json:"created_at,omitempty"`
 	Seed      bool         `yaml:"seed,omitempty" json:"seed,omitempty"` // true if migrated from config gateway_keys
-	// TokenQuota caps this key's TOTAL tokens across every model it may use.
-	// 0 = unlimited. Period/Hours define the reset window, exactly like
-	// ModelScope: "" never resets, "hour"/"week"/"month" fixed windows,
-	// "nhour" uses Hours.
-	TokenQuota int64  `yaml:"token_quota,omitempty" json:"token_quota,omitempty"`
-	Period     string `yaml:"period,omitempty" json:"period,omitempty"`
-	Hours      int64  `yaml:"hours,omitempty" json:"hours,omitempty"`
-	// ReqQuota caps the number of requests per reset window; 0 = unlimited.
-	// RPM covers short bursts; this covers sustained volume.
-	ReqQuota int64 `yaml:"req_quota,omitempty" json:"req_quota,omitempty"`
 }
 
-// KeyQuota is the set of key-wide caps accepted by the admin API. It is a
-// separate struct so a partial update can be expressed as a pointer (nil =
-// "leave the stored caps alone") instead of zero values meaning "clear".
+// KeyQuota is retained only to carry a scope entry's caps through the admin
+// API. Quotas are per model, never per key: there is deliberately no key-wide
+// total, so exhausting one model's budget never blocks the others.
 type KeyQuota struct {
 	TokenQuota int64  `json:"token_quota"`
 	ReqQuota   int64  `json:"req_quota"`
@@ -397,14 +387,6 @@ type KeyQuota struct {
 	Hours      int64  `json:"hours"`
 }
 
-// ApplyQuota writes the caps onto a key record.
-func (k *GWKey) ApplyQuota(q KeyQuota) {
-	k.TokenQuota = q.TokenQuota
-	k.ReqQuota = q.ReqQuota
-	k.Period = q.Period
-	k.Hours = q.Hours
-}
-
 // NormalizeRole defaults an empty role to "user", so a key can never end up in
 // a state where no role means "neither admin nor user".
 func NormalizeRole(role string) string {
@@ -453,15 +435,18 @@ func ValidatePeriod(period string, hours int64) error {
 	return fmt.Errorf("period must be one of \"\", hour, week, month, nhour (got %q)", period)
 }
 
-// ModelScope is one allowed model for a key, or one AUTO scheduling slot,
-// with an optional token quota and reset period. TokenQuota 0 = unlimited;
-// Period "" = never resets; "hour"/"week"/"month" are fixed windows; "nhour"
-// uses Hours as the window length in hours.
+// ModelScope is one allowed model for a key, or one AUTO scheduling slot.
+// Its TokenQuota and ReqQuota cap THAT entry only, independently of every
+// other entry on the same key: a model that runs out of budget stops being
+// served while the key's other models keep working. TokenQuota 0 / ReqQuota 0
+// = unlimited. Period "" = never resets; "hour"/"week"/"month" are fixed
+// windows; "nhour" uses Hours.
 type ModelScope struct {
 	Model      string `yaml:"model" json:"model"`
 	Source     string `yaml:"source,omitempty" json:"source,omitempty"` // optional: pin to one upstream source; "" = any source
 	Tier       int    `yaml:"tier,omitempty" json:"tier,omitempty"`
 	TokenQuota int64  `yaml:"token_quota" json:"token_quota"`
+	ReqQuota   int64  `yaml:"req_quota,omitempty" json:"req_quota,omitempty"`
 	Period     string `yaml:"period,omitempty" json:"period,omitempty"`
 	Hours      int64  `yaml:"hours,omitempty" json:"hours,omitempty"`
 }
@@ -480,6 +465,7 @@ func (m *ModelScope) UnmarshalJSON(b []byte) error {
 		Source     string `json:"source"`
 		Tier       int    `json:"tier"`
 		TokenQuota int64  `json:"token_quota"`
+		ReqQuota   int64  `json:"req_quota"`
 		Period     string `json:"period"`
 		Hours      int64  `json:"hours"`
 	}
@@ -490,6 +476,7 @@ func (m *ModelScope) UnmarshalJSON(b []byte) error {
 	m.Source = o.Source
 	m.Tier = o.Tier
 	m.TokenQuota = o.TokenQuota
+	m.ReqQuota = o.ReqQuota
 	m.Period = o.Period
 	m.Hours = o.Hours
 	return nil