Adds background task for invoice processing

Implements a background task that listens for paid invoices
and automatically records them in the accounting system. This
ensures payments are captured even if the user closes their
browser before the client-side polling detects the payment.

Introduces a new `get_journal_entry_by_reference` function to
improve idempotency when recording payments.

views_api.py

@@ -591,7 +591,7 @@ async def api_generate_payment_invoice(
         amount=data.amount,
         memo=f"Payment from user {target_user_id[:8]} to Castle",
         unit="sat",
-        extra={"user_id": target_user_id, "type": "castle_payment"},
+        extra={"tag": "castle", "user_id": target_user_id},
     )
 
     payment = await create_payment_request(castle_wallet_id, invoice_data)
@@ -648,6 +648,18 @@ async def api_record_payment(
             detail="Payment metadata missing user_id. Cannot determine which user to credit.",
         )
 
+    # Check if payment already recorded (idempotency)
+    from .crud import get_journal_entry_by_reference
+    existing = await get_journal_entry_by_reference(data.payment_hash)
+    if existing:
+        # Payment already recorded, return existing entry
+        balance = await get_user_balance(target_user_id)
+        return {
+            "journal_entry_id": existing.id,
+            "new_balance": balance.balance,
+            "message": "Payment already recorded",
+        }
+
     # Convert amount from millisatoshis to satoshis
     amount_sats = payment.amount // 1000