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/service/ops_realtime_models.go

@@ -37,6 +37,17 @@ type AccountConcurrencyInfo struct {
 	WaitingInQueue int64   `json:"waiting_in_queue"`
 }
 
+// UserConcurrencyInfo represents real-time concurrency usage for a single user.
+type UserConcurrencyInfo struct {
+	UserID         int64   `json:"user_id"`
+	UserEmail      string  `json:"user_email"`
+	Username       string  `json:"username"`
+	CurrentInUse   int64   `json:"current_in_use"`
+	MaxCapacity    int64   `json:"max_capacity"`
+	LoadPercentage float64 `json:"load_percentage"`
+	WaitingInQueue int64   `json:"waiting_in_queue"`
+}
+
 // PlatformAvailability aggregates account availability by platform.
 type PlatformAvailability struct {
 	Platform            string           `json:"platform"`