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/payment/internal/handlers/webhooks.go

@@ -57,9 +57,20 @@ func (d *Dispatcher) Enqueue(ctx context.Context, client *models.ClientApp, t *m
 	}
 	body, _ := json.Marshal(payload)
 	sig := signing.Sign(client.Secret, body)
-	if _, err := d.store.EnqueueWebhook(ctx, t.ID, *client.WebhookURL, body, sig); err != nil {
+	id, err := d.store.EnqueueWebhook(ctx, t.ID, *client.WebhookURL, body, sig)
+	if err != nil {
 		log.Printf("webhook enqueue failed for txn %s: %v", t.ID, err)
+		return
 	}
+	// Best-effort immediate delivery so the client credits near-instantly; the
+	// retry loop (Run) still covers it if this attempt fails.
+	go func() {
+		bg, cancel := context.WithTimeout(context.Background(), 20*time.Second)
+		defer cancel()
+		if w, err := d.store.GetWebhook(bg, id); err == nil && w != nil {
+			d.deliver(bg, w)
+		}
+	}()
 }
 
 // Run starts the delivery loop until ctx is cancelled.