feat(rpm): RPM 限流模块优化

P0:
- rpm_override 嵌入 Auth Cache Snapshot，消除每请求 DB 查询 (snapshot v6→v7)
- 429 RPM 响应返回 Retry-After 头（当前分钟剩余秒数）

P1:
- ClearAll 按钮直连 DELETE API，带 loading 防重复
- 新增 GET /admin/users/:id/rpm-status 管理员 RPM 用量查询端点

优化:
- checkRPM 从级联互斥改为并行取最严，user.rpm_limit 作为全局硬上限始终生效
- Override/Group 变更后自动失效 auth cache
- fail-open 语义不变，Redis 故障不阻塞业务

backend/ent/migrate/schema.go

@@ -491,6 +491,7 @@ var (
 		{Name: "require_privacy_set", Type: field.TypeBool, Default: false},
 		{Name: "default_mapped_model", Type: field.TypeString, Size: 100, Default: ""},
 		{Name: "messages_dispatch_model_config", Type: field.TypeJSON, SchemaType: map[string]string{"postgres": "jsonb"}},
+		{Name: "rpm_limit", Type: field.TypeInt, Default: 0},
 	}
 	// GroupsTable holds the schema information for the "groups" table.
 	GroupsTable = &schema.Table{
@@ -1276,7 +1277,7 @@ var (
 		{Name: "totp_secret_encrypted", Type: field.TypeString, Nullable: true, SchemaType: map[string]string{"postgres": "text"}},
 		{Name: "totp_enabled", Type: field.TypeBool, Default: false},
 		{Name: "totp_enabled_at", Type: field.TypeTime, Nullable: true},
-		{Name: "signup_source", Type: field.TypeString, Size: 20, Default: "email"},
+		{Name: "signup_source", Type: field.TypeString, Default: "email"},
 		{Name: "last_login_at", Type: field.TypeTime, Nullable: true, SchemaType: map[string]string{"postgres": "timestamptz"}},
 		{Name: "last_active_at", Type: field.TypeTime, Nullable: true, SchemaType: map[string]string{"postgres": "timestamptz"}},
 		{Name: "balance_notify_enabled", Type: field.TypeBool, Default: true},
@@ -1284,6 +1285,7 @@ var (
 		{Name: "balance_notify_threshold", Type: field.TypeFloat64, Nullable: true, SchemaType: map[string]string{"postgres": "decimal(20,8)"}},
 		{Name: "balance_notify_extra_emails", Type: field.TypeString, Default: "[]", SchemaType: map[string]string{"postgres": "text"}},
 		{Name: "total_recharged", Type: field.TypeFloat64, Default: 0, SchemaType: map[string]string{"postgres": "decimal(20,8)"}},
+		{Name: "rpm_limit", Type: field.TypeInt, Default: 0},
 	}
 	// UsersTable holds the schema information for the "users" table.
 	UsersTable = &schema.Table{