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/service/user_service_test.go

@@ -96,6 +96,18 @@ func (m *mockBillingCache) UpdateSubscriptionUsage(context.Context, int64, int64
 func (m *mockBillingCache) InvalidateSubscriptionCache(context.Context, int64, int64) error {
 	return nil
 }
+func (m *mockBillingCache) GetAPIKeyRateLimit(context.Context, int64) (*APIKeyRateLimitCacheData, error) {
+	return nil, nil
+}
+func (m *mockBillingCache) SetAPIKeyRateLimit(context.Context, int64, *APIKeyRateLimitCacheData) error {
+	return nil
+}
+func (m *mockBillingCache) UpdateAPIKeyRateLimitUsage(context.Context, int64, float64) error {
+	return nil
+}
+func (m *mockBillingCache) InvalidateAPIKeyRateLimit(context.Context, int64) error {
+	return nil
+}
 
 // --- 测试 ---