The backend had zero tests. This adds 33, all pure (no DB, no network), plus the
harness to run them in the gate:
- backend/Dockerfile.test: the app's deps + pinned pytest/ruff, code bind-mounted at
run time so it always tests the working tree. Isolated from the prod Dockerfile, so
the published image never carries test tooling.
- requirements-dev.txt: pytest==8.4.2, ruff==0.15.21 (the version the gate already
runs) — the backend lane is now pinned, not just host-global.
- tests: normalize_title (de-shout, hashtag strip, emoji drop, trilingual letters
survive); storage sanitize/rel_path/download_filename and the safe_abs_path
traversal guard (../ and absolute-path escapes rejected — verified by mutation);
links HMAC grants (round-trip, wrong-token, tampered sig/exp, expiry) + is_expired;
a DB-free app-assembly smoke (every router wires up, OpenAPI generates).
DB-backed router smoke (auth/feed/downloads) needs a test Postgres + migrations —
deferred. useCardPager needs hook-test infra (jsdom/renderHook) — deferred.
