fix: document read fails with 403 when auth is disabled (#4623)

* fix: document read fails with 403 when auth is disabled

Add _auth_disabled() bypass in _verify_doc_owner() and the
/api/documents/{session_id} route guard so documents remain accessible
in single-user / no-auth mode.

Minimal change: only adds the auth-disabled check alongside existing
403 raises — preserves existing formatting and line endings.

* refactor: hoist _auth_disabled import to module level

Address reviewer feedback on PR #4623 — no circular import exists
(src.auth_helpers only imports stdlib + fastapi), so the inline
imports are unnecessary. Moves the import to module top in both
document_helpers.py and document_routes.py.

* test: add regression tests for auth-disabled document access (PR #4623)

routes/document_routes.py

@@ -10,7 +10,7 @@ from fastapi import APIRouter, HTTPException, Query, Request, UploadFile, File,
 from sqlalchemy import case, func, or_
 from core.database import SessionLocal, Document, DocumentVersion
 from core.database import Session as DbSession
-from src.auth_helpers import get_current_user
+from src.auth_helpers import get_current_user, _auth_disabled
 from src.constants import MAIL_ATTACHMENTS_DIR
 
 logger = logging.getLogger(__name__)
@@ -388,7 +388,8 @@ def setup_document_routes(session_manager, upload_handler=None) -> APIRouter:
         db = SessionLocal()
         try:
             if not user:
-                raise HTTPException(403, "Authentication required")
+                if not _auth_disabled():
+                    raise HTTPException(403, "Authentication required")
             # v2 review HIGH-9: raise 403 explicitly when the caller
             # can't see this session, instead of returning [] which the
             # UI treats identically to "no docs" and silently masks