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/handler/dto/mappers.go

@@ -212,17 +212,6 @@ func AccountFromServiceShallow(a *service.Account) *Account {
 		}
 	}
 
-	if scopeLimits := a.GetAntigravityScopeRateLimits(); len(scopeLimits) > 0 {
-		out.ScopeRateLimits = make(map[string]ScopeRateLimitInfo, len(scopeLimits))
-		now := time.Now()
-		for scope, remainingSec := range scopeLimits {
-			out.ScopeRateLimits[scope] = ScopeRateLimitInfo{
-				ResetAt:      now.Add(time.Duration(remainingSec) * time.Second),
-				RemainingSec: remainingSec,
-			}
-		}
-	}
-
 	return out
 }