fix: resolve CI lint errors and test compilation failures for rate limit feature

- Fix errcheck: properly handle rows.Close() error via named return + defer closure
- Fix gofmt: auto-format billing_cache.go, api_key_service.go, billing_cache_service.go
- Add missing rate limit interface methods to 4 test stubs (GetRateLimitData, IncrementRateLimitUsage, ResetRateLimitWindows)
- Fix NewBillingCacheService calls missing the new apiKeyRepo parameter

backend/internal/repository/api_key_repo.go

@@ -477,7 +477,7 @@ func (r *apiKeyRepository) ResetRateLimitWindows(ctx context.Context, id int64)
 }
 
 // GetRateLimitData returns the current rate limit usage and window start times for an API key.
-func (r *apiKeyRepository) GetRateLimitData(ctx context.Context, id int64) (*service.APIKeyRateLimitData, error) {
+func (r *apiKeyRepository) GetRateLimitData(ctx context.Context, id int64) (result *service.APIKeyRateLimitData, err error) {
 	rows, err := r.sql.QueryContext(ctx, `
 		SELECT usage_5h, usage_1d, usage_7d, window_5h_start, window_1d_start, window_7d_start
 		FROM api_keys
@@ -486,7 +486,11 @@ func (r *apiKeyRepository) GetRateLimitData(ctx context.Context, id int64) (*ser
 	if err != nil {
 		return nil, err
 	}
-	defer rows.Close()
+	defer func() {
+		if closeErr := rows.Close(); closeErr != nil && err == nil {
+			err = closeErr
+		}
+	}()
 	if !rows.Next() {
 		return nil, service.ErrAPIKeyNotFound
 	}