feat: add support for paying amountless BOLT11 invoices

Add ability to pay BOLT11 invoices that don't have an embedded amount
by specifying the amount at payment time via the `amount_msat` parameter.

Changes:
- Add `Feature.amountless_invoice` to wallet base class for capability detection
- Update `Wallet.pay_invoice()` signature with optional `amount_msat` parameter
- Implement amountless invoice support in LND REST and LND gRPC wallets
- Update payment service layer to validate and pass through amount_msat
- Add `amount_msat` field to CreateInvoice API model
- Update all wallet implementations with new method signature
- Add tests for amountless invoice payment flow

Usage: POST /api/v1/payments with `amount_msat` when paying amountless invoices

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests/unit/test_pay_invoice.py

@@ -42,7 +42,7 @@ async def test_amountless_invoice(to_wallet: Wallet):
         "73aym6ynrdl9nkzqnag49vt3sjjn8qdfq5cr6ha0vrdz5c5r3v4aghndly0hplmv"
         "6hjxepwp93cq398l3s"
     )
-    with pytest.raises(PaymentError, match="Amountless invoices not supported."):
+    with pytest.raises(PaymentError, match="Amount required for amountless invoices."):
         await pay_invoice(
             wallet_id=to_wallet.id,
             payment_request=zero_amount_invoice,