real middleman: BTCPay payment gate + webhook + Printful adapter + checkout redirect

backend/app/schemas.py

@@ -131,6 +131,8 @@ class OrderOut(ORMModel):
     status: str
     tracking: Optional[str] = None
     carrier: Optional[str] = None
+    btcpay_invoice_id: Optional[str] = None
+    supplier_order_ref: Optional[str] = None
     created_at: Optional[datetime] = None
     updated_at: Optional[datetime] = None
 
@@ -147,6 +149,11 @@ class CustomerOrderOut(ORMModel):
     updated_at: Optional[datetime] = None
 
 
+class CheckoutResponse(BaseModel):
+    order: OrderOut
+    checkout_url: Optional[str] = None
+
+
 class TrackingIn(BaseModel):
     tracking: str
     carrier: Optional[str] = None