feat(notifications+admin): SMS (Kavenegar) + Email (SMTP) channels & templates

Backend (notification-svc):
- channel_config table (per-tenant Kavenegar + SMTP settings) + migration 18
- sender pkg: Kavenegar SMS client + SMTP mailer (STARTTLS / implicit TLS), stdlib only
- endpoints: GET/PUT /v1/channels[/:channel], POST /v1/sms/send, POST /v1/email/send
  (template + {{var}} rendering); deliveries logged
- seeded 3 Persian email templates: welcome / account_verification / promotion
- gateway routes /v1/{channels,sms,email}/* → notification

Admin UI:
- /admin/messaging: SMS + Email provider config cards, test-send, email template editor
- nav link + fa/en labels

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

services/gateway/cmd/server/main.go

@@ -139,6 +139,9 @@ func main() {
 	// ── Notification Service ──────────────────────────────────────────────────
 	v1.Any("/notifications/*path",          apiRL, auth, notification.Handler())
 	v1.Any("/notification-templates/*path", apiRL, auth, notification.Handler())
+	v1.Any("/channels/*path",               apiRL, auth, notification.Handler())
+	v1.Any("/sms/*path",                    apiRL, auth, notification.Handler())
+	v1.Any("/email/*path",                  apiRL, auth, notification.Handler())
 
 	log.Printf("gateway listening on :%s", port)
 	if err := r.Run(":" + port); err != nil {