feat: nip-47 nwc wallet service engine with per-connection budgets

src/lnurlcash/nwc/budget.ts

@@ -0,0 +1,37 @@
+// The per-connection NWC budget: max msat per rolling period, persisted
+// in storage/nwcConnections so a restart (or a browser crash mid-session)
+// doesn't reset a client's allowance. The service serializes pay_invoice
+// per connection through its request queue, which is what makes the
+// check + record below atomic enough: no two pays of one connection ever
+// run this concurrently.
+
+import type {NwcConnectionRecord} from '../storage/nwcConnections'
+import {persistNwcConnection} from '../storage/nwcConnections'
+
+export const budgetRemainingMsat = (
+  record: NwcConnectionRecord,
+  nowMs: number
+): number => {
+  const {maxMsat, periodMs} = record.budget
+  if (nowMs - record.spent.periodStart >= periodMs) return maxMsat
+  return Math.max(0, maxMsat - record.spent.msat)
+}
+
+// rolls the period when it expired, then adds the spend; persists (the
+// caller's queue serialized this read-modify-write)
+export const recordSpend = (
+  record: NwcConnectionRecord,
+  amountMsat: number,
+  nowMs: number
+): NwcConnectionRecord => {
+  const expired = nowMs - record.spent.periodStart >= record.budget.periodMs
+  return persistNwcConnection({
+    ...record,
+    spent: expired
+      ? {periodStart: nowMs, msat: amountMsat}
+      : {
+          periodStart: record.spent.periodStart,
+          msat: record.spent.msat + amountMsat
+        }
+  })
+}