feat: per-user channel config for Google Chat and Nextcloud Talk

- New endpoints: POST /channels/google-chat/{username} and /webhook/nextcloud/{username}
- Channel secrets/config live in home/{username}/channels.json (gitignored)
- auth_utils: get_user_channels() helper reads channels.json
- Both routers load persona, audience/secret, backend, timeout per user;
  set_context() wires the correct persona before building the system prompt
- Removed server-level channel settings from config.py and .env —
  no user gets a channel until they create their own channels.json
- .gitignore: home/**/channels.json added

To migrate: update Google Chat Add-on webhook URL to /channels/google-chat/{username}
and re-register NC Talk bot at /webhook/nextcloud/{username}

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

.gitignore

@@ -15,6 +15,7 @@ home/**/sessions/
 home/**/auth.json
 home/**/invite.json
 home/**/profile.json
+home/**/channels.json
 
 # Syncthing Metadata
 .stfolder/