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>

.env.example

@@ -8,6 +8,9 @@ MP_SMTP_TLS_KEY=sans:mailpit.local
 MP_WEBHOOK_URL=http://sneaky_mon_webhook:8088/hook
 
 # SNEAKYMON VARS
-GOTIFY_URL=https://yourgotify.server.com
-GOTIFY_TOKEN=YourTokenForGotify
-MAILPIT_API=http://mailpit:8025 # <-- this points to above, it can even be the IP if you want.
+MAILPIT_API=http://mailpit:8025 # <-- this points to above, it can even be the IP if you want.
+
+# Ntfy notification service
+NTFY_URL=https://ntfy.sh
+NTFY_TOKEN=YourNtfyAccessToken
+NTFY_TOPIC=your-topic-name