fix: correct migration 061 checksum and add missing BillingCache mock methods

- Fix fileChecksum for 061 migration: use TrimSpace hash (66207e7a) instead
  of raw sha256sum (97bdd9a3), matching the actual runtime computation
- Add 222b4a09 as accepted DB checksum for 061 migration
- Add missing GetAPIKeyRateLimit/SetAPIKeyRateLimit/UpdateAPIKeyRateLimitUsage/
  InvalidateAPIKeyRateLimit methods to mock BillingCache in test stubs
- Fix NewBillingCacheService call in singleflight test (add apiKeyRepo param)

backend/internal/repository/api_key_repo.go

@@ -21,7 +21,11 @@ type apiKeyRepository struct {
 }
 
 func NewAPIKeyRepository(client *dbent.Client, sqlDB *sql.DB) service.APIKeyRepository {
-	return &apiKeyRepository{client: client, sql: sqlDB}
+	return newAPIKeyRepositoryWithSQL(client, sqlDB)
+}
+
+func newAPIKeyRepositoryWithSQL(client *dbent.Client, sqlq sqlExecutor) *apiKeyRepository {
+	return &apiKeyRepository{client: client, sql: sqlq}
 }
 
 func (r *apiKeyRepository) activeQuery() *dbent.APIKeyQuery {