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>

backend/app/config.py

@@ -54,6 +54,14 @@ class Settings(BaseSettings):
     # pytrends
     PYTRENDS_ENABLED: bool = True
 
+    # SMTP (Email notifications)
+    SMTP_HOST: str = ""
+    SMTP_PORT: int = 587
+    SMTP_USER: str = ""
+    SMTP_PASSWORD: str = ""
+    SMTP_FROM: str = ""       # Defaults to SMTP_USER if blank
+    SMTP_STARTTLS: bool = True
+
 
 @lru_cache
 def get_settings() -> Settings: