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/config/settings.py

@@ -37,6 +37,10 @@ class AppSettings(BaseSettings):
     rag_chunk_overlap: int = Field(validation_alias="STIRLING_RAG_CHUNK_OVERLAP")
     rag_default_top_k: int = Field(validation_alias="STIRLING_RAG_TOP_K")
     rag_max_searches: int = Field(validation_alias="STIRLING_RAG_MAX_SEARCHES")
+    documents_reaper_interval_seconds: int = Field(
+        default=900,
+        validation_alias="STIRLING_DOCUMENTS_REAPER_INTERVAL_SECONDS",
+    )
 
     # Chunked reasoner settings (whole-document map-reduce).
     chunked_reasoner_chars_per_slice: int = Field(validation_alias="STIRLING_CHUNKED_REASONER_CHARS_PER_SLICE")