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/scheduler_snapshot_service.go

@@ -151,6 +151,14 @@ func (s *SchedulerSnapshotService) GetAccount(ctx context.Context, accountID int
 	return s.accountRepo.GetByID(fallbackCtx, accountID)
 }
 
+// UpdateAccountInCache 立即更新 Redis 中单个账号的数据（用于模型限流后立即生效）
+func (s *SchedulerSnapshotService) UpdateAccountInCache(ctx context.Context, account *Account) error {
+	if s.cache == nil || account == nil {
+		return nil
+	}
+	return s.cache.SetAccount(ctx, account)
+}
+
 func (s *SchedulerSnapshotService) runInitialRebuild() {
 	if s.cache == nil {
 		return