migrate from gotify to ntfy with image attachment support

- Replace GotifyNotifier with NtfyNotifier for push notifications
- Add support for sending email image attachments to ntfy
- Add NTFY_URL, NTFY_TOKEN, NTFY_TOPIC environment variables
- Add get_mailpit_image() and get_mailpit_image_thumb() helpers
- Sanitize message content for HTTP headers (remove newlines)
- Remove all gotify references

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

app/dev_main.py

@@ -5,5 +5,5 @@ load_dotenv()
 
 from main import handle_hook
 
-msg_id = "jtdx9og7NAQ6LuVvEmRL6t"
+msg_id = "42EJJktKsUUaH7zjQ6YoKY"
 handle_hook(msg_id)