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/repository/account_repo.go

@@ -798,53 +798,6 @@ func (r *accountRepository) SetRateLimited(ctx context.Context, id int64, resetA
 	return nil
 }
 
-func (r *accountRepository) SetAntigravityQuotaScopeLimit(ctx context.Context, id int64, scope service.AntigravityQuotaScope, resetAt time.Time) error {
-	now := time.Now().UTC()
-	payload := map[string]string{
-		"rate_limited_at":     now.Format(time.RFC3339),
-		"rate_limit_reset_at": resetAt.UTC().Format(time.RFC3339),
-	}
-	raw, err := json.Marshal(payload)
-	if err != nil {
-		return err
-	}
-
-	scopeKey := string(scope)
-	client := clientFromContext(ctx, r.client)
-	result, err := client.ExecContext(
-		ctx,
-		`UPDATE accounts SET
-			extra = jsonb_set(
-				jsonb_set(COALESCE(extra, '{}'::jsonb), '{antigravity_quota_scopes}'::text[], COALESCE(extra->'antigravity_quota_scopes', '{}'::jsonb), true),
-				ARRAY['antigravity_quota_scopes', $1]::text[],
-				$2::jsonb,
-				true
-			),
-			updated_at = NOW(),
-			last_used_at = NOW()
-		WHERE id = $3 AND deleted_at IS NULL`,
-		scopeKey,
-		raw,
-		id,
-	)
-	if err != nil {
-		return err
-	}
-
-	affected, err := result.RowsAffected()
-	if err != nil {
-		return err
-	}
-	if affected == 0 {
-		return service.ErrAccountNotFound
-	}
-
-	if err := enqueueSchedulerOutbox(ctx, r.sql, service.SchedulerOutboxEventAccountChanged, &id, nil, nil); err != nil {
-		log.Printf("[SchedulerOutbox] enqueue quota scope failed: account=%d err=%v", id, err)
-	}
-	return nil
-}
-
 func (r *accountRepository) SetModelRateLimit(ctx context.Context, id int64, scope string, resetAt time.Time) error {
 	if scope == "" {
 		return nil