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>

Dockerfile

@@ -35,7 +35,7 @@ RUN pip install -U setuptools cryptography==37.0.4 poetry==1.1.7
 COPY backend/pyproject.toml backend/poetry.lock ./
 RUN poetry config virtualenvs.path /venv
 RUN poetry install --no-dev --no-ansi --no-interaction
-RUN poetry run pip install -U setuptools psycopg2-binary $EXTRA_DEPS
+RUN poetry run pip install -U setuptools psycopg2-binary python-jose[cryptography] $EXTRA_DEPS
 
 COPY backend/manage.py backend/gunicorn.conf.py ./
 COPY backend/tabby tabby