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/api/v1/admin.py

@@ -8,6 +8,7 @@ from app.services.admin import AdminService
 from app.services.translation_quota import TranslationQuotaService
 from app.services.certification import CertificationService
 from app.services.invoice import InvoiceService
+from app.services.payment import PaymentService
 from app.api.v1.deps import get_current_user
 
 router = APIRouter()
@@ -274,3 +275,41 @@ async def admin_process_invoice(
     if not result:
         raise HTTPException(status_code=404, detail="Invoice not found")
     return result
+
+
+@router.get("/payments")
+async def admin_list_payments(
+    page: int = Query(1, ge=1),
+    size: int = Query(20, ge=1, le=100),
+    gateway: str = Query(default=""),
+    status: str = Query(default=""),
+    user_id: str = Query(default=""),
+    _: dict = Depends(require_admin),
+    db: AsyncSession = Depends(get_db),
+):
+    svc = PaymentService(db)
+    return await svc.admin_list_payments(page, size, gateway, status, user_id)
+
+
+@router.get("/payments/stats")
+async def admin_payment_stats(
+    _: dict = Depends(require_admin),
+    db: AsyncSession = Depends(get_db),
+):
+    svc = PaymentService(db)
+    return await svc.admin_payment_stats()
+
+
+@router.post("/payments/refund")
+async def admin_refund(
+    data: dict,
+    _: dict = Depends(require_admin),
+    db: AsyncSession = Depends(get_db),
+):
+    order_no = data.get("order_no", "")
+    reason = data.get("reason", "")
+    svc = PaymentService(db)
+    try:
+        return await svc.admin_refund(order_no, reason)
+    except ValueError as e:
+        raise HTTPException(status_code=400, detail=str(e))