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>

services/identity/FlatRender.IdentitySvc/Controllers/PaymentsController.cs

@@ -65,6 +65,24 @@ public class PaymentsController(IPaymentService paymentService) : ControllerBase
         return Redirect(frontendUrl);
     }
 
+    // ── FlatRender Pay broker flow ────────────────────────────────────────────────
+
+    /// <summary>
+    /// GET /v1/payments/callback/broker?payment_id={p}&amp;id={brokerTxn}&amp;status=&amp;sign=
+    /// The broker redirects the user's browser here after payment. We confirm the
+    /// transaction authoritatively via the broker inquiry API, activate the plan,
+    /// then redirect to the frontend result page. Public (no JWT).
+    /// </summary>
+    [AllowAnonymous]
+    [HttpGet("payments/callback/broker")]
+    public async Task<IActionResult> BrokerCallback(
+        [FromQuery] Guid payment_id,
+        [FromQuery] string? id)
+    {
+        var frontendUrl = await paymentService.HandleBrokerCallbackAsync(payment_id, id ?? "");
+        return Redirect(frontendUrl);
+    }
+
     // ── SnapPay flow ──────────────────────────────────────────────────────────────
 
     /// <summary>