refactor: replace direct WeChat/Alipay with unified pay-api gateway

Switch from direct WeChat Pay / Alipay integrations to the unified
宇之然 pay-api gateway (HMAC-SHA256 auth). Removes wechat_pay.py,
keeps PaymentGateway abstraction, adds UnifiedPayService. Simplifies
payment.py create_order to {plan, pay_type} params. Single webhook
endpoint replaces separate WeChat/Alipay notify handlers.

backend/app/models/__init__.py

@@ -18,26 +18,23 @@ from .referral import ReferralCode, Referral
 from .search_provider import SearchProvider
 from .discovery_record import DiscoveryRecord
 from .ai_provider import AIProvider
+from .payment_transaction import PaymentTransaction
 
 __all__ = [
     "User", "Product",
     "Customer", "Conversation", "Message",
     "Quotation", "QuotationItem",
-    "CorpusEntry",
-    "Team", "TeamMember",
-    "UsageLog",
-    "Notification",
-    "Feedback",
     "Subscription",
+    "CorpusEntry",
+    "Notification",
+    "Team", "TeamMember",
+    "Feedback",
     "PreferenceAnalysis", "MarketingEffect",
-    "Device",
     "FollowupStrategy", "FollowupLog",
-    "SystemConfig",
-    "TranslationQuota",
-    "Certification", "CertType", "CertStatus",
-    "Invoice", "InvoiceType", "InvoiceStatus",
+    "Certification", "Invoice", "InvoiceType", "InvoiceStatus",
     "ReferralCode", "Referral",
     "SearchProvider",
     "DiscoveryRecord",
     "AIProvider",
+    "PaymentTransaction",
 ]