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>

cortex/config.py

@@ -45,21 +45,6 @@ class Settings(BaseSettings):
     timeout_gemini: int = 120   # frequently slow under load
     timeout_local: int = 300    # local models may need to load first
 
-    # Google Chat
-    # JWT audience (aud) claim to verify on inbound webhook requests.
-    # Google Chat sets aud = the Google Cloud project number (e.g. "741112865538").
-    # Set to "" to disable verification (dev/testing only).
-    google_chat_audience: str = ""
-    # Google Chat must receive a response within 30s or shows an error to the user
-    google_chat_timeout: int = 25
-    # Backend forced for Google Chat — Claude is more reliable within the 25s deadline
-    google_chat_backend: str = "claude"
-
-    # Nextcloud Talk bot
-    nextcloud_url: str = "https://cloud.dgrzone.com"
-    nextcloud_talk_bot_secret: str = ""   # set in .env
-    nextcloud_talk_timeout: int = 55
-
     # Auto-distillation schedule — override in .env
     # AUTO_DISTILL=false disables entirely
     scheduler_timezone: str = "America/New_York"  # IANA tz — override in .env if needed