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>

messages/en.json

@@ -323,7 +323,8 @@
       "templates": "Templates",
       "media": "Media",
       "discounts": "Discounts",
-      "siteSettings": "Settings"
+      "siteSettings": "Settings",
+      "messaging": "Messaging"
     },
     "appAdminNodesPage": {
       "title": "Render Nodes",

messages/fa.json

@@ -323,7 +323,8 @@
       "templates": "قالب‌ها",
       "media": "رسانه",
       "discounts": "تخفیف‌ها",
-      "siteSettings": "تنظیمات سایت"
+      "siteSettings": "تنظیمات سایت",
+      "messaging": "پیام‌رسانی"
     },
     "appAdminNodesPage": {
       "title": "نودهای رندر",