api + web: pull album art on first view, not first CDJ load

[vynulldev]

What & why

Newly added tracks showed no artwork in the web UI until a CDJ loaded them. The add path only reads the tag-embedded picture, which misses art ffmpeg can see, and the artwork endpoint's lazy extraction (one probe on first request, then cached for good) was unreachable from the UI: every render site gated the img on art_id > 0, so the request that would have triggered extraction was never made. Loading the track on a deck ran analysis, whose artwork write-back set art_id, and only then did the UI show art, a chicken-and-egg the CDJ happened to break.

The track payload now carries art_checked (the server-side "has this file ever been probed" flag), and a shared artHTML helper, replacing three copy-pasted render sites (table ART column, mobile card, detail drawer), renders the img optimistically whenever the file has never been probed: the first view of a new track fires the artwork request, the endpoint extracts synchronously, and the same response delivers the image. Tracks probed and found artless go straight to the placeholder, so there is no repeated 404 traffic, and an onerror fallback covers the artless and concurrent-probe cases.

Verified in the browser against a fresh library: a never-probed track with embedded art shows it on first open of the detail drawer with art_id and art_checked persisted behind it, an artless track degrades to the placeholder, and after its probe it makes no further artwork requests across a reload.

Hardware testing

• Tested on: N/A: no deck-facing change. The dbserver artwork path CDJs use is untouched; this only adds a field to the HTTP track payload and changes how the web UI requests art.

Checklist

• go build ./..., go vet ./..., and go test ./... pass
• gofmt -l . is clean
• New source files carry an SPDX header (GPL-3.0-or-later) (n/a, no new files)
• Tested on real hardware (deck + firmware noted above), or this change doesn't affect deck behaviour
• I agree my contribution is licensed under the project's GPLv3