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/models.py

@@ -86,6 +86,9 @@ class Settings(Base):
     notification_time = Column(String(5), default="07:00")
     timezone = Column(String(50), default="UTC")
     location_name = Column(String(100))
+    ntfy_username = Column(String(200))
+    ntfy_password = Column(String(200))
+    ntfy_api_key = Column(String(200))
 
 
 class NotificationLog(Base):