feat: dynamic auth providers + Auth0 support

- Add /api/1/auth/providers endpoint that returns only configured providers
- Frontend fetches available providers dynamically instead of hardcoding
- Only providers with credentials set (KEY + SECRET) appear as login options
- Add Auth0 as a supported authentication provider
- Add python-jose[cryptography] dependency for Auth0 JWT verification
- Show helpful message when no providers are configured

This makes the login page modular - administrators only see buttons for
providers they've actually configured, avoiding confusion from dead buttons.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

backend/tabby/app/api/__init__.py

@@ -11,6 +11,7 @@ router.register(
 
 urlpatterns = [
     path("api/1/auth/logout", auth.LogoutView.as_view()),
+    path("api/1/auth/providers", auth.ProvidersView.as_view()),
     path("api/1/user", user.UserViewSet.as_view({"get": "retrieve", "put": "update"})),
     path(
         "api/1/gateways/choose",