Set up document management for Stirling Engine (#6476)

# Description of Changes
Change Stirling Engine to support deleting documents automatically. This
happens both on user logout and after an amount of time specified by the
Java when ingesting a document (allowing for personal documents to have
short lifetimes but org documents to be left in the db with no expiry
date). Also sets up an [ACL
policy](https://en.wikipedia.org/wiki/Access-control_list) for the
documents so the database knows which users have access to which
documents. This is not fully implemented in the Java, so currently all
docs are treated as having a single owner, the uploader, but
theoretically when we need to support org storage, we shouldn't need to
change the db schema.

engine/tests/test_pdf_question_agent.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+from collections.abc import Iterator
 from dataclasses import replace
 
 import pytest
@@ -19,9 +20,25 @@ from stirling.contracts import (
     SupportedCapability,
 )
 from stirling.documents import Document, DocumentService, SqliteVecStore
-from stirling.models import FileId
+from stirling.models import FileId, OwnerId, PrincipalId, UserId
+from stirling.services import current_user_id
 from stirling.services.runtime import AppRuntime
 
+USER = UserId("test-user")
+OWNER = OwnerId("test-user")
+OWNER_PRINCIPALS = [PrincipalId("test-user")]
+
+
+@pytest.fixture(autouse=True)
+def _set_user_context() -> Iterator[None]:
+    """Set current_user_id to a fixed test user for every test in this module so
+    agent code calling require_current_user_id() doesn't fail closed."""
+    token = current_user_id.set(USER)
+    try:
+        yield
+    finally:
+        current_user_id.reset(token)
+
 
 class StubEmbedder:
     """Deterministic embeddings so RAG lookups work in tests without network."""
@@ -94,6 +111,9 @@ async def test_reports_only_missing_files(runtime_with_stub_rag: AppRuntime) ->
         FileId("present-id"),
         [PageText(page_number=1, text="Invoice total: 120.00.")],
         source="present.pdf",
+        owner_id=OWNER,
+        read_principals=OWNER_PRINCIPALS,
+        expires_at=None,
     )
     agent = PdfQuestionAgent(runtime_with_stub_rag)
 
@@ -111,6 +131,9 @@ async def test_returns_grounded_answer_when_all_files_ingested(runtime_with_stub
         FileId("invoice-id"),
         [PageText(page_number=1, text="Invoice total: 120.00.")],
         source="invoice.pdf",
+        owner_id=OWNER,
+        read_principals=OWNER_PRINCIPALS,
+        expires_at=None,
     )
     agent = StubPdfQuestionAgent(
         runtime_with_stub_rag,
@@ -142,6 +165,9 @@ async def test_returns_not_found_when_answer_not_in_doc(runtime_with_stub_rag: A
         FileId("shipping-id"),
         [PageText(page_number=1, text="This page contains only a shipping address.")],
         source="shipping.pdf",
+        owner_id=OWNER,
+        read_principals=OWNER_PRINCIPALS,
+        expires_at=None,
     )
     agent = StubPdfQuestionAgent(
         runtime_with_stub_rag,