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_contracts.py

@@ -77,17 +77,20 @@ def test_pdf_question_answer_defaults_evidence_list() -> None:
 def test_app_settings_accepts_model_configuration() -> None:
     from pathlib import Path
 
-    from stirling.config import RagBackend
+    from stirling.config import DocumentsBackend
 
     settings = AppSettings(
         smart_model_name="claude-sonnet-4-5-20250929",
         fast_model_name="claude-haiku-4-5-20251001",
         smart_model_max_tokens=8192,
         fast_model_max_tokens=2048,
-        rag_backend=RagBackend.SQLITE,
+        model_max_concurrency=32,
+        documents_backend=DocumentsBackend.SQLITE,
         rag_embedding_model="voyageai:voyage-4",
-        rag_store_path=Path(":memory:"),
-        rag_pgvector_dsn="",
+        documents_sqlite_path=Path(":memory:"),
+        documents_pgvector_dsn="",
+        documents_pgvector_pool_min_size=1,
+        documents_pgvector_pool_max_size=10,
         rag_chunk_size=512,
         rag_chunk_overlap=64,
         rag_default_top_k=5,
@@ -98,6 +101,7 @@ def test_app_settings_accepts_model_configuration() -> None:
         chunked_reasoner_notes_char_budget=250_000,
         max_pages=200,
         max_characters=200_000,
+        require_user_id=False,
         posthog_enabled=False,
         posthog_api_key="",
         posthog_host="https://eu.i.posthog.com",