- C1: remove downloads/formats.py target_ext() — defined but never called (the
worker derives the real extension from the produced file's suffix).
- C2: the download-root containment+existence guard was copy-pasted 6× across the
file-serving endpoints (routes/downloads.py ×3, routes/public.py ×3). Extract
storage.safe_abs_path(root, rel) -> Path|None so this security-sensitive check
lives in one place; behavior identical (same containment test + messages). The
extraction also made `pathlib.Path` unused in both route modules (removed).