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/db/db.go

@@ -292,6 +292,20 @@ func (s *Store) EnqueueWebhook(ctx context.Context, txnID uuid.UUID, url string,
 	return id, err
 }
 
+// GetWebhook loads a single delivery row (used for the inline immediate attempt).
+func (s *Store) GetWebhook(ctx context.Context, id uuid.UUID) (*WebhookDelivery, error) {
+	var w WebhookDelivery
+	err := s.pool.QueryRow(ctx, `
+		SELECT id, transaction_id, url, payload, signature, attempts
+		FROM payment.webhook_deliveries
+		WHERE id = $1 AND delivered = FALSE`, id).Scan(
+		&w.ID, &w.TransactionID, &w.URL, &w.Payload, &w.Signature, &w.Attempts)
+	if errors.Is(err, pgx.ErrNoRows) {
+		return nil, ErrNotFound
+	}
+	return &w, err
+}
+
 // ClaimDueWebhooks returns undelivered webhooks whose next_attempt_at has passed.
 func (s *Store) ClaimDueWebhooks(ctx context.Context, limit int) ([]*WebhookDelivery, error) {
 	rows, err := s.pool.Query(ctx, `