* 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)
Rudra Sarker