fix(bluebubbles): auto-register webhook with BlueBubbles server on connect

**Problem:**
The BlueBubbles iMessage gateway was not receiving incoming messages even though:
1. BlueBubbles Server was properly configured and running
2. Hermes gateway started without errors
3. Webhook listener was started on the configured port

The root cause was that the BlueBubbles adapter only started a local webhook
listener but never registered the webhook URL with the BlueBubbles server via
the API. Without registration, the server doesn't know where to send events.

**Fix:**
1. Added _register_webhook() method that POSTs to /api/v1/webhook with the
   listener URL and event types (new-message, updated-message, message)
2. Added _unregister_webhook() method for clean shutdown
3. Both methods handle the case where webhook listens on 0.0.0.0/127.0.0.1
   by using 'localhost' as the external hostname
4. Fixed documentation: 'hermes gateway logs' → 'hermes logs gateway'

**API Reference:**
https://docs.bluebubbles.app/server/developer-guides/rest-api-and-webhooks

**Testing:**
- Webhook registration is now automatic when gateway starts
- Failed registration logs a warning but doesn't prevent startup
- Clean shutdown unregisters the webhook

Closes: iMessage gateway not working issue

website/docs/user-guide/messaging/bluebubbles.md

@@ -135,8 +135,9 @@ Without the Private API, basic text messaging and media still work.
 ### Messages not arriving
 - Check that the webhook is registered in BlueBubbles Server → Settings → API → Webhooks
 - Verify the webhook URL is reachable from the Mac
-- Check `hermes gateway logs` for webhook errors
+- Check `hermes logs gateway` for webhook errors (or `hermes logs -f` to follow in real-time)
 
 ### "Private API helper not connected"
 - Install the Private API helper: [docs.bluebubbles.app](https://docs.bluebubbles.app/helper-bundle/installation)
 - Basic messaging works without it — only reactions, typing, and read receipts require it
+