Add ntfy authentication support (username/password and API key)

- Settings table gets ntfy_username, ntfy_password, ntfy_api_key columns
- Backend applies Basic or Bearer auth header when sending notifications
- Settings page UI lets you toggle between no auth, basic, or token auth
- Masked credential display on load to avoid exposing stored secrets
- README updated with auth modes documentation

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

backend/schemas.py

@@ -78,6 +78,9 @@ class SettingsUpdate(BaseModel):
     notification_time: Optional[str] = "07:00"
     timezone: Optional[str] = "UTC"
     location_name: Optional[str] = None
+    ntfy_username: Optional[str] = None
+    ntfy_password: Optional[str] = None
+    ntfy_api_key: Optional[str] = None
 
 
 class SettingsOut(SettingsUpdate):