feat(antigravity): comprehensive enhancements - model mapping, rate limiting, scheduling & ops

Key changes:
- Upgrade model mapping: Opus 4.5 → Opus 4.6-thinking with precise matching
- Unified rate limiting: scope-level → model-level with Redis snapshot sync
- Load-balanced scheduling by call count with smart retry mechanism
- Force cache billing support
- Model identity injection in prompts with leak prevention
- Thinking mode auto-handling (max_tokens/budget_tokens fix)
- Frontend: whitelist mode toggle, model mapping validation, status indicators
- Gemini session fallback with Redis Trie O(L) matching
- Ops: enhanced concurrency monitoring, account availability, retry logic
- Migration scripts: 049-051 for model mapping unification

backend/internal/server/routes/admin.go

@@ -78,6 +78,7 @@ func registerOpsRoutes(admin *gin.RouterGroup, h *handler.Handlers) {
 	{
 		// Realtime ops signals
 		ops.GET("/concurrency", h.Admin.Ops.GetConcurrencyStats)
+		ops.GET("/user-concurrency", h.Admin.Ops.GetUserConcurrencyStats)
 		ops.GET("/account-availability", h.Admin.Ops.GetAccountAvailability)
 		ops.GET("/realtime-traffic", h.Admin.Ops.GetRealtimeTrafficSummary)
 
@@ -228,6 +229,9 @@ func registerAccountRoutes(admin *gin.RouterGroup, h *handler.Handlers) {
 		accounts.POST("/batch-refresh-tier", h.Admin.Account.BatchRefreshTier)
 		accounts.POST("/bulk-update", h.Admin.Account.BulkUpdate)
 
+		// Antigravity 默认模型映射
+		accounts.GET("/antigravity/default-model-mapping", h.Admin.Account.GetAntigravityDefaultModelMapping)
+
 		// Claude OAuth routes
 		accounts.POST("/generate-auth-url", h.Admin.OAuth.GenerateAuthURL)
 		accounts.POST("/generate-setup-token-url", h.Admin.OAuth.GenerateSetupTokenURL)