fix(admin): fix greyed-out QR login toggle on admin connections page

- Add `qr_login_enabled` boolean config field (default True) to app/config.py
- Add `qr_login_enabled` entry to settings metadata in app/utils/settings_service.py
- Fix app/views/settings.py to use `qr_login_enabled` directly instead of
  deriving enablement status from qr_login_challenge_ttl_seconds (integer TTL)
- Fix admin_connections.html: remove hardcoded `disabled` attribute from the
  Mobile Phone Upload toggle and wire up onchange handler so toggling actually
  persists the setting via toggleSetting('qr_login_enabled', this.checked)
- Gate all three QR auth API endpoints on settings.qr_login_enabled so the
  feature is actually disabled when the toggle is turned off

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>
Agent-Logs-Url: https://github.com/christianlouis/DocuElevate/sessions/8421cdb2-d92d-4b83-9cda-c44983d35173

app/views/settings.py

@@ -464,7 +464,7 @@ async def connections_page(request: Request, db: Session = Depends(get_db)):
 
         # Feature toggles
         sso_auto_login = _is_truthy(_get_effective("sso_auto_login"))
-        qr_login_enabled = _is_truthy(_get_effective("qr_login_challenge_ttl_seconds"))
+        qr_login_enabled = _is_truthy(_get_effective("qr_login_enabled"))
         frontend_url_configured = bool(_get_effective("public_base_url"))
 
         return templates.TemplateResponse(