Audit path_to_rel usage for multi-user roots

[blinkerbit]

Source: gitlab-issues-export.md (Issue 9)

Weight: 3

Problem / context

codereview.md: when root_dir is omitted, path_to_rel may use global ROOT_DIR instead of per-user root, mis-labeling audit paths.

Suggested approach / acceptance criteria

• Grep call sites under aird/handlers/ and services; pass explicit get_user_root / share root where needed.
• Add tests for multi-user audit labels if missing.