feat(payments): route coin purchases through FlatRender Pay broker

ZarinPal only accepts callbacks on pay.flatrender.ir, so bargevasat
pays through the shared broker and is credited via a signed webhook.

- FlatPayService: broker client (HMAC-signed /v1/pay/request) + webhook
  signature verification + in-memory idempotency guard.
- Program.cs: /api/coins/pay/request prefers the broker when configured
  (FlatPay__ApiKey/Secret set), else the legacy direct ZarinPal path;
  new public POST /api/coins/pay/webhook verifies the HMAC and credits
  coins from the echoed metadata (idempotent).
- appsettings + docker-compose: FlatPay config (empty ⇒ legacy path).
- web: recognise the broker's ?status=Paid return + re-refresh profile
  (coins are credited server-side via webhook).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docker-compose.yml

@@ -56,6 +56,13 @@ services:
       Zarinpal__Sandbox: ${ZARINPAL_SANDBOX:-true}
       Zarinpal__CallbackUrl: ${ZARINPAL_CALLBACK_URL:-http://localhost:1505/api/coins/pay/callback}
       Zarinpal__ClientReturnUrl: ${ZARINPAL_CLIENT_RETURN_URL:-http://localhost:1500}
+      # FlatRender Pay broker (pay.flatrender.ir): shared ZarinPal via the single
+      # verified domain. Set FLATPAY_API_KEY + FLATPAY_SECRET to route through it
+      # (issued in FlatRender admin → پرداخت). Empty ⇒ legacy direct ZarinPal above.
+      FlatPay__BaseUrl: ${FLATPAY_BASE_URL:-https://pay.flatrender.ir}
+      FlatPay__ApiKey: ${FLATPAY_API_KEY:-}
+      FlatPay__Secret: ${FLATPAY_SECRET:-}
+      FlatPay__ReturnUrl: ${FLATPAY_RETURN_URL:-https://bargevasat.ir/?pay=done}
       # Store in-app billing verification (Cafe Bazaar / Myket) — fill from panels.
       Iab__PackageName: ${IAB_PACKAGE_NAME:-com.bargevasat.app}
       Iab__BazaarClientId: ${IAB_BAZAAR_CLIENT_ID:-}