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/settings.py

@@ -140,10 +140,15 @@ AUTHENTICATION_BACKENDS = (
     "social_core.backends.azuread.AzureADOAuth2",
     "social_core.backends.microsoft.MicrosoftOAuth2",
     "social_core.backends.google.GoogleOAuth2",
+    "social_core.backends.auth0.Auth0OAuth2",
     "django.contrib.auth.backends.ModelBackend",
 )
 
 SOCIAL_AUTH_GITHUB_SCOPE = ["read:user", "user:email"]
+SOCIAL_AUTH_AUTH0_SCOPE = ["openid", "profile", "email"]
+SOCIAL_AUTH_AUTH0_EXTRA_DATA = ["id_token"]
+# Auth0 uses RS256 for ID tokens
+SOCIAL_AUTH_ALLOWED_REDIRECT_HOSTS = ["*"]
 SOCIAL_AUTH_PIPELINE = (
     "social_core.pipeline.social_auth.social_details",
     "social_core.pipeline.social_auth.social_uid",
@@ -180,6 +185,9 @@ for key in [
     "SOCIAL_AUTH_GOOGLE_OAUTH2_SECRET",
     "SOCIAL_AUTH_MICROSOFT_GRAPH_KEY",
     "SOCIAL_AUTH_MICROSOFT_GRAPH_SECRET",
+    "SOCIAL_AUTH_AUTH0_DOMAIN",
+    "SOCIAL_AUTH_AUTH0_KEY",
+    "SOCIAL_AUTH_AUTH0_SECRET",
     "CONNECTION_GATEWAY_AUTH_CA",
     "CONNECTION_GATEWAY_AUTH_CERTIFICATE",
     "CONNECTION_GATEWAY_AUTH_KEY",