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

.taskfiles/engine.yml

@@ -43,7 +43,7 @@ tasks:
     env:
       PYTHONUNBUFFERED: "1"
     cmds:
-      - uv run uvicorn stirling.api.app:app --host 0.0.0.0 --port {{.PORT}}
+      - uv run uvicorn stirling.api.app:app --host 0.0.0.0 --port {{.PORT}} --workers "${STIRLING_ENGINE_WORKERS:-4}"
 
   dev:
     desc: "Start engine dev server with hot reload"