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/tests/test_stirling_api.py

@@ -214,6 +214,26 @@ def test_pdf_edit_route() -> None:
     assert response.json()["outcome"] == "cannot_do"
 
 
+def test_routes_require_user_id_when_enforced() -> None:
+    """With STIRLING_REQUIRE_USER_ID on, an identity-less request is rejected
+    at the boundary before any handler runs; supplying X-User-Id is accepted.
+    The other route tests in this module run with the flag off and cover the
+    identity-less self-hosted path."""
+    app.dependency_overrides[load_settings] = lambda: build_app_settings().model_copy(update={"require_user_id": True})
+    try:
+        anonymous = client.post("/api/v1/pdf/edit", json={"userMessage": "rotate this"})
+        identified = client.post(
+            "/api/v1/pdf/edit",
+            json={"userMessage": "rotate this"},
+            headers={"X-User-Id": "alice"},
+        )
+    finally:
+        app.dependency_overrides[load_settings] = build_app_settings
+
+    assert anonymous.status_code == 401
+    assert identified.status_code == 200
+
+
 def test_pdf_questions_route() -> None:
     response = client.post(
         "/api/v1/pdf/questions",