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_integration_test.go

@@ -26,7 +26,7 @@ func (s *APIKeyRepoSuite) SetupTest() {
 	s.ctx = context.Background()
 	tx := testEntTx(s.T())
 	s.client = tx.Client()
-	s.repo = NewAPIKeyRepository(s.client).(*apiKeyRepository)
+	s.repo = newAPIKeyRepositoryWithSQL(s.client, tx)
 }
 
 func TestAPIKeyRepoSuite(t *testing.T) {
@@ -421,7 +421,7 @@ func (s *APIKeyRepoSuite) TestIncrementQuotaUsed_DeletedKey() {
 // 注意：此测试使用 testEntClient（非事务隔离），数据会真正写入数据库。
 func TestIncrementQuotaUsed_Concurrent(t *testing.T) {
 	client := testEntClient(t)
-	repo := NewAPIKeyRepository(client).(*apiKeyRepository)
+	repo := NewAPIKeyRepository(client, integrationDB).(*apiKeyRepository)
 	ctx := context.Background()
 
 	// 创建测试用户和 API Key