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/api_key_service_cache_test.go

@@ -106,6 +106,15 @@ func (s *authRepoStub) IncrementQuotaUsed(ctx context.Context, id int64, amount
 func (s *authRepoStub) UpdateLastUsed(ctx context.Context, id int64, usedAt time.Time) error {
 	panic("unexpected UpdateLastUsed call")
 }
+func (s *authRepoStub) IncrementRateLimitUsage(ctx context.Context, id int64, cost float64) error {
+	panic("unexpected IncrementRateLimitUsage call")
+}
+func (s *authRepoStub) ResetRateLimitWindows(ctx context.Context, id int64) error {
+	panic("unexpected ResetRateLimitWindows call")
+}
+func (s *authRepoStub) GetRateLimitData(ctx context.Context, id int64) (*APIKeyRateLimitData, error) {
+	panic("unexpected GetRateLimitData call")
+}
 
 type authCacheStub struct {
 	getAuthCache   func(ctx context.Context, key string) (*APIKeyAuthCacheEntry, error)