fix(agent): resolve missing imports — jose, auth config, AUTH_SECRET_KEY

- Added python-jose[cryptography] to requirements-agent.txt
- Made sqlalchemy.orm.Session and TokenBlacklist imports lazy in jwt.py
  so decode_token works without pulling ORM models into agent image
- Added src/core/auth/__init__.py, config.py, jwt.py to agent Dockerfile COPY
- Added AUTH_SECRET_KEY env var to agent compose (reads from JWT_SECRET)

docker-compose.yml

@@ -60,6 +60,7 @@ services:
       LLM_MODEL: ${LLM_MODEL:-gpt-4o}
       FASTAPI_URL: http://backend:8000
       JWT_SECRET: ${JWT_SECRET:?JWT_SECRET must be set — crash-early, no default fallback}
+      AUTH_SECRET_KEY: ${JWT_SECRET:?JWT_SECRET must be set — crash-early, no default fallback}
       SERVICE_JWT: ${SERVICE_JWT:-agent-service-secret}
       DATABASE_URL: postgresql+psycopg2://postgres:postgres@db:5432/ss_tools
       GRADIO_SERVER_PORT: 7860