refactor: replace scope-level rate limiting with model-level rate limiting

Merge functional changes from develop branch:
- Remove AntigravityQuotaScope system (claude/gemini_text/gemini_image)
- Replace with per-model rate limiting using resolveAntigravityModelKey
- Remove model load statistics (IncrModelCallCount/GetModelLoadBatch)
- Simplify account selection to unified priority→load→LRU algorithm
- Remove SetAntigravityQuotaScopeLimit from AccountRepository
- Clean up scope-related UI indicators and API fields

backend/internal/service/ops_account_availability.go

@@ -66,7 +66,6 @@ func (s *OpsService) GetAccountAvailabilityStats(ctx context.Context, platformFi
 		}
 
 		isAvailable := acc.Status == StatusActive && acc.Schedulable && !isRateLimited && !isOverloaded && !isTempUnsched
-		scopeRateLimits := acc.GetAntigravityScopeRateLimits()
 
 		if acc.Platform != "" {
 			if _, ok := platform[acc.Platform]; !ok {
@@ -85,14 +84,6 @@ func (s *OpsService) GetAccountAvailabilityStats(ctx context.Context, platformFi
 			if hasError {
 				p.ErrorCount++
 			}
-			if len(scopeRateLimits) > 0 {
-				if p.ScopeRateLimitCount == nil {
-					p.ScopeRateLimitCount = make(map[string]int64)
-				}
-				for scope := range scopeRateLimits {
-					p.ScopeRateLimitCount[scope]++
-				}
-			}
 		}
 
 		for _, grp := range acc.Groups {
@@ -117,14 +108,6 @@ func (s *OpsService) GetAccountAvailabilityStats(ctx context.Context, platformFi
 			if hasError {
 				g.ErrorCount++
 			}
-			if len(scopeRateLimits) > 0 {
-				if g.ScopeRateLimitCount == nil {
-					g.ScopeRateLimitCount = make(map[string]int64)
-				}
-				for scope := range scopeRateLimits {
-					g.ScopeRateLimitCount[scope]++
-				}
-			}
 		}
 
 		displayGroupID := int64(0)
@@ -157,9 +140,6 @@ func (s *OpsService) GetAccountAvailabilityStats(ctx context.Context, platformFi
 				item.RateLimitRemainingSec = &remainingSec
 			}
 		}
-		if len(scopeRateLimits) > 0 {
-			item.ScopeRateLimits = scopeRateLimits
-		}
 		if isOverloaded && acc.OverloadUntil != nil {
 			item.OverloadUntil = acc.OverloadUntil
 			remainingSec := int64(time.Until(*acc.OverloadUntil).Seconds())