feat(admin): AI model catalog — provider config, model routing, test connection
- app_settings.py: add ai_providers[], active_provider, active_main/mini_model to DEFAULTS
- admin.py: GET/PUT /api/admin/ai-config (API key masked on read, preserved if not updated)
POST /api/admin/ai-config/test (latency + connection check)
- AIConfigTab.tsx: provider dropdown, endpoint/key fields, models table with role+enabled toggles,
main/mini routing selects, "Test connection" with live latency feedback
- Admin.tsx: add "AI Config" tab
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>