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/schemas/schemas.py

@@ -35,6 +35,8 @@ class NotificationSettingsResponse(BaseModel):
     ntfy_enabled: bool = False
     rss_enabled: bool = False
     rss_token: Optional[str] = None
+    email_enabled: bool = False
+    email_address: str = ""
     # Digest
     digest_enabled: bool = False
     digest_frequency: str = "daily"  # daily | weekly
@@ -55,6 +57,8 @@ class NotificationSettingsUpdate(BaseModel):
     ntfy_password: Optional[str] = None
     ntfy_enabled: Optional[bool] = None
     rss_enabled: Optional[bool] = None
+    email_enabled: Optional[bool] = None
+    email_address: Optional[str] = None
     digest_enabled: Optional[bool] = None
     digest_frequency: Optional[str] = None
     quiet_hours_start: Optional[int] = None