Integrate BTCPay Server: auto-settling invoice deposits

- lib/btcpay.ts: Greenfield API client (createBtcPayInvoice, getBtcPayInvoiceStatus)
- /api/btcpay/invoice: creates per-deposit invoice (USD amount + handle in metadata)
- /api/btcpay/status/[id]: polls invoice status + returns BTC address/amount
- add-funds page: BTCPay tab with amount picker, live address display,
  10s auto-poll, settlement auto-credits localStorage ledger
- WalletContext: creditBtcPayInvoice() with duplicate-invoice guard
- .env.example: BTCPAY_URL, BTCPAY_API_KEY, BTCPAY_STORE_ID, NEXT_PUBLIC_BTCPAY_ENABLED

Made-with: Cursor

app/api/btcpay/status/[invoiceId]/route.ts

@@ -0,0 +1,32 @@
+import { NextResponse } from "next/server";
+import { getBtcPayInvoiceStatus, isBtcPayConfigured } from "@/lib/btcpay";
+
+export async function GET(
+  _req: Request,
+  { params }: { params: { invoiceId: string } },
+) {
+  if (!isBtcPayConfigured()) {
+    return NextResponse.json({ ok: false, error: "BTCPay not configured" }, { status: 503 });
+  }
+
+  const { invoiceId } = params;
+  if (!invoiceId || invoiceId.length < 4) {
+    return NextResponse.json({ ok: false, error: "Invalid invoice ID" }, { status: 400 });
+  }
+
+  const result = await getBtcPayInvoiceStatus(invoiceId);
+  if (!result.ok) {
+    return NextResponse.json({ ok: false, error: result.error }, { status: 502 });
+  }
+
+  return NextResponse.json({
+    ok: true,
+    status: result.status,
+    usdAmount: result.usdAmount,
+    btcAddress: result.paymentMethod?.destination ?? null,
+    btcAmount: result.paymentMethod?.due ?? null,
+    btcRate: result.paymentMethod?.rate ?? null,
+    totalPaid: result.paymentMethod?.totalPaid ?? null,
+    settled: result.status === "Settled",
+  });
+}