refactor: add status column to apipayments (#2537)

* refactor: add status column to apipayments

keep track of the payment status with an enum and persist it as string
to db. `pending`, `success`, `failed`.

- database migration
- remove deleting of payments, failed payments stay

lnbits/core/views/public_api.py

@@ -20,7 +20,8 @@ async def api_public_payment_longpolling(payment_hash):
         raise HTTPException(
             status_code=HTTPStatus.NOT_FOUND, detail="Payment does not exist."
         )
-    elif not payment.pending:
+    # TODO: refactor to use PaymentState
+    if payment.success:
         return {"status": "paid"}
 
     try: