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",

backend/tabby/app/api/auth.py

@@ -1,9 +1,73 @@
+from django.conf import settings
 from django.contrib.auth import logout
 from rest_framework.response import Response
 from rest_framework.views import APIView
 
 
+# Provider configuration: maps backend names to their display info
+# and the environment variable prefix used to detect if they're configured
+PROVIDER_CONFIG = {
+    'github': {
+        'name': 'GitHub',
+        'icon': 'github',
+        'cls': 'btn-primary',
+        'env_prefix': 'SOCIAL_AUTH_GITHUB',
+    },
+    'gitlab': {
+        'name': 'GitLab',
+        'icon': 'gitlab',
+        'cls': 'btn-warning',
+        'env_prefix': 'SOCIAL_AUTH_GITLAB',
+    },
+    'google-oauth2': {
+        'name': 'Google',
+        'icon': 'google',
+        'cls': 'btn-secondary',
+        'env_prefix': 'SOCIAL_AUTH_GOOGLE_OAUTH2',
+    },
+    'microsoft-graph': {
+        'name': 'Microsoft',
+        'icon': 'microsoft',
+        'cls': 'btn-light',
+        'env_prefix': 'SOCIAL_AUTH_MICROSOFT_GRAPH',
+    },
+    'auth0': {
+        'name': 'Auth0',
+        'icon': 'key',  # Using key icon as Auth0 doesn't have a FA brand icon
+        'cls': 'btn-dark',
+        'env_prefix': 'SOCIAL_AUTH_AUTH0',
+    },
+}
+
+
+def is_provider_configured(env_prefix: str) -> bool:
+    """Check if a provider has both KEY and SECRET configured."""
+    key = getattr(settings, f'{env_prefix}_KEY', None)
+    secret = getattr(settings, f'{env_prefix}_SECRET', None)
+    # For Auth0, also need DOMAIN
+    if env_prefix == 'SOCIAL_AUTH_AUTH0':
+        domain = getattr(settings, f'{env_prefix}_DOMAIN', None)
+        return bool(key and secret and domain)
+    return bool(key and secret)
+
+
 class LogoutView(APIView):
     def post(self, request, format=None):
         logout(request)
         return Response(None)
+
+
+class ProvidersView(APIView):
+    """Returns list of configured authentication providers."""
+
+    def get(self, request, format=None):
+        providers = []
+        for provider_id, config in PROVIDER_CONFIG.items():
+            if is_provider_configured(config['env_prefix']):
+                providers.append({
+                    'id': provider_id,
+                    'name': config['name'],
+                    'icon': config['icon'],
+                    'cls': config['cls'],
+                })
+        return Response(providers)