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/src/stirling/models/__init__.py

@@ -1,12 +1,15 @@
 from . import tool_models
-from .base import ApiModel, FileId
+from .base import ApiModel, FileId, OwnerId, PrincipalId, UserId
 from .tool_models import OPERATIONS, ParamToolModel, ToolEndpoint
 
 __all__ = [
     "ApiModel",
     "FileId",
     "OPERATIONS",
+    "OwnerId",
     "ParamToolModel",
+    "PrincipalId",
     "ToolEndpoint",
+    "UserId",
     "tool_models",
 ]

engine/src/stirling/models/base.py

@@ -5,11 +5,18 @@ from typing import NewType
 from pydantic import BaseModel, ConfigDict
 from pydantic.alias_generators import to_camel
 
-# Stable, opaque identifier for a file supplied by the caller. Owned by the caller's
-# ID strategy (content hash, filesystem path, etc.) and used as the RAG collection key
-# throughout the engine.
+# Stable, opaque identifier for a file supplied by the caller
 FileId = NewType("FileId", str)
 
+# Stable, opaque identifier for the calling user
+UserId = NewType("UserId", str)
+
+# Tenant that owns a document
+OwnerId = NewType("OwnerId", str)
+
+# An entity that can hold permissions on a document
+PrincipalId = NewType("PrincipalId", str)
+
 
 class ApiModel(BaseModel):
     """Base for every contract model crossing a service boundary."""