feat(payment): route FlatRender plan purchases through the broker

- identity: when FlatPay (broker) is configured, InitiateZarinPalAsync
  routes through pay.flatrender.ir instead of calling ZarinPal directly;
  new HandleBrokerCallbackAsync confirms the payment via the broker
  inquiry API (authoritative, not trusting the redirect) and activates
  the plan. New public endpoint GET /v1/payments/callback/broker
  (already public at the gateway via /callback/*). Env-gated — empty
  FlatPay__ApiKey keeps the legacy direct-ZarinPal path.
- broker: deliver webhooks inline on enqueue (best-effort) in addition
  to the retry loop, so clients credit near-instantly (db.GetWebhook +
  goroutine kick).
- compose + ENV_FILE: FlatPay__* for identity (FLATPAY_FLATRENDER_*).

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

docker-compose.v2.yml

@@ -89,6 +89,13 @@ services:
       ZarinPal__MerchantId:   "${ZARINPAL_MERCHANT_ID:-}"
       ZarinPal__CallbackUrl:  "${ZARINPAL_CALLBACK_URL:-http://localhost:8080/v1/payments/callback/zarinpal}"
       ZarinPal__Sandbox:      "${ZARINPAL_SANDBOX:-true}"
+      # FlatRender Pay broker — when ApiKey+Secret are set, plan purchases route
+      # through pay.flatrender.ir (the single ZarinPal-verified domain) instead of a
+      # direct ZarinPal call. ReturnBase = this identity service's public base.
+      FlatPay__BaseUrl:       "${FLATPAY_BASE_URL:-https://pay.flatrender.ir}"
+      FlatPay__ApiKey:        "${FLATPAY_FLATRENDER_API_KEY:-}"
+      FlatPay__Secret:        "${FLATPAY_FLATRENDER_SECRET:-}"
+      FlatPay__ReturnBase:    "${FLATPAY_RETURN_BASE:-https://api.flatrender.ir}"
       Stripe__SecretKey:      "${STRIPE_SECRET_KEY:-}"
       Stripe__WebhookSecret:  "${STRIPE_WEBHOOK_SECRET:-}"
       SnapPay__ClientId:      "${SNAPPAY_CLIENT_ID:-}"