feat: fiat payments (revolut)

ref: #24

lnvps_db/migrations/20250310153305_fiat_payment.sql

@@ -0,0 +1,6 @@
+alter table vm_payment
+    add column currency varchar(5) not null default 'BTC',
+    add column payment_method      smallint unsigned not null default 0,
+    add column external_id varchar(255),
+    change invoice external_data varchar (4096) NOT NULL,
+    drop column settle_index;