feat: email notifications with tabbed channel UI (v0.9.10)

Add email as a second notification channel alongside ntfy:
- Tabbed channel selector: ntfy | Email | Telegram (coming soon) | Discord (coming soon)
- Active channel shown with green status dot on tab
- Email tab: address input, Save & Enable, Test, Disable — same UX pattern as ntfy
- Backend: SMTP config in settings (SMTP_HOST/PORT/USER/PASSWORD/FROM/STARTTLS)
- Dispatcher: _send_email() helper wired into dispatch_notifications
- POST /api/notifications/test/email endpoint with descriptive error messages
- Email fires in same window as ntfy (respects quiet hours / digest hold)
- Telegram and Discord tabs show coming-soon banners with planned feature description
- .env.example documents all SMTP settings with provider examples

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.env.example

@@ -59,3 +59,17 @@ NEWSAPI_KEY=
 
 # ─── Google Trends ────────────────────────────────────────────────────────────
 PYTRENDS_ENABLED=true
+
+# ─── SMTP (Email Notifications) ───────────────────────────────────────────────
+# Leave SMTP_HOST blank to disable email notifications entirely.
+# Supports any standard SMTP server (Gmail, Outlook, Postmark, Mailgun, etc.)
+# Gmail example: HOST=smtp.gmail.com PORT=587 USER=you@gmail.com (use App Password)
+# Postmark example: HOST=smtp.postmarkapp.com PORT=587 USER=<api-token> PASSWORD=<api-token>
+SMTP_HOST=
+SMTP_PORT=587
+SMTP_USER=
+SMTP_PASSWORD=
+# From address shown in the email header — defaults to SMTP_USER if blank
+SMTP_FROM=
+# Set to false only if your SMTP server uses implicit TLS on port 465
+SMTP_STARTTLS=true