The sync editor renders server covers as <img src="/api/navidrome/cover/{id}">,
but no Flask route ever served that path — so every Navidrome cover 404'd, on
every album, art or not. The source (left) side then went blank too: a source
row with no native art (e.g. YouTube, which provides none at mirror time) falls
back to borrowing the matched server track's cover — i.e. that same dead route.
So both sides collapsed to nothing.
Fix:
- New NavidromeClient.build_cover_art_url(cover_id) — builds the absolute,
Subsonic-authenticated getCoverArt URL (base_url + token/salt), keeping
credentials server-side. Uses a FIXED cover-art salt so the URL is
deterministic for a given (server, password, cover_id): a rotating salt (as
in _generate_auth_params) would make every request a unique URL → image-cache
miss every time + a dead, never-reused cache row per fetch. Token auth doesn't
require a unique salt, and the password is never exposed (only its salted md5).
- New route /api/navidrome/cover/<cover_id> — resolves that URL and streams the
image through the shared image cache (same pattern as /api/image-proxy), with
a private max-age so the browser caches by the stable route URL.
Effect: server side works for any album that has art in Navidrome; matched
source rows with no native art now borrow the (now-working) server cover.
Unmatched YouTube rows stay blank — no image exists anywhere to show.
Tests: tests/test_navidrome_cover_url.py (8) — URL structure + salted-token auth
(never the raw password), determinism (same id -> same URL so the cache hits;
different id/password -> different URL), optional size, and the not-connected /
no-id / no-credentials guards.
Caveats: not executed against a live Navidrome (no server in CI) — the URL
builder is unit-tested; the route's cache→HTTP→bytes round-trip is read-verified
only. Scope is the sync editor's Navidrome route; Plex/Jellyfin server-cover
branches and any other modals using a different mechanism are untouched.
