Enables admin to generate invoices for users

Allows administrators to generate payment invoices on behalf of specific users.
This is useful for handling settlement invoices in certain scenarios.

The changes include:
- Adding a `user_id` field to the generate payment invoice request model
- Updating the API endpoint to accept the `user_id` parameter
- Implementing checks to ensure only superusers can generate invoices for other users
- Updating the memo and extra data to reflect the target user

models.py

@@ -173,6 +173,7 @@ class GeneratePaymentInvoice(BaseModel):
     """Generate payment invoice request"""
 
     amount: int
+    user_id: Optional[str] = None  # For admin-generated settlement invoices
 
 
 class RecordPayment(BaseModel):