Improvements to Stirling Engine to prepare for SaaS release (#6603)

# Description of Changes
- Use pool for postgres connections
- Add ability to require user ID to be set on API calls to the engine
- Add process-wide concurrency cap on AI access (in addition to existing
user caps)
- Allow number of workers (threads) to be specified for stirling engine
- Update env var names to reflect that the DB is not just for RAG

engine/Dockerfile

@@ -26,6 +26,7 @@ COPY .taskfiles/ ./.taskfiles/
 
 ENV PATH="/app/engine/.venv/bin:$PATH"
 ENV PYTHONUNBUFFERED=1
+ENV STIRLING_ENGINE_WORKERS=4
 
 EXPOSE 5001