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/service/admin_service_apikey_test.go

@@ -127,6 +127,15 @@ func (s *apiKeyRepoStubForGroupUpdate) IncrementQuotaUsed(context.Context, int64
 func (s *apiKeyRepoStubForGroupUpdate) UpdateLastUsed(context.Context, int64, time.Time) error {
 	panic("unexpected")
 }
+func (s *apiKeyRepoStubForGroupUpdate) IncrementRateLimitUsage(context.Context, int64, float64) error {
+	panic("unexpected")
+}
+func (s *apiKeyRepoStubForGroupUpdate) ResetRateLimitWindows(context.Context, int64) error {
+	panic("unexpected")
+}
+func (s *apiKeyRepoStubForGroupUpdate) GetRateLimitData(context.Context, int64) (*APIKeyRateLimitData, error) {
+	panic("unexpected")
+}
 
 // groupRepoStubForGroupUpdate implements GroupRepository for AdminUpdateAPIKeyGroupID tests.
 type groupRepoStubForGroupUpdate struct {