Posters asked for ?w= come smaller for every kind of title, not only games - #199

Closes #178
Refs #89

What changes

GET /api/items/{id}/poster?w=… now makes a smaller copy for every kind of title. Before, only games got one; films, episodes, music, books and the rest got the original file whatever w said.

• It's the games' code (GameMeta.PosterThumb), unchanged in what it accepts: only a cache: picture or a picture inside a library folder (Store.LibraryFile), never a remote address, only up to 640 px, snapped to the thumbnail widths, decoded only after its header says it's a picture of at most 16 megapixels. The serving path after it is the same serveArtSrc, so the picture rules (no SVG, sandboxed) still apply.
• At most two copies are made at once (was half the processors). Each decode holds the picture twice, up to 128 MB for the largest allowed, so a page of new posters on a 20-thread machine could have needed over a gigabyte.
• The small copies now have a limit: 512 MB, trimmed oldest first with the other caches after a scan (at most once a day). The real artwork in the same folder is never trimmed: trimCache takes a name prefix now, and only gamethumb- files count.
• The files keep their gamethumb- names, so the games' copies already made stay in use.

The web app doesn't ask with ?w= yet (posterUrl in web/src/api.ts and music's coverUrl): that's #90's, noted for that chat in the shared notes. Until then this changes what the server can do, not what the web app gets. Lists that already ask with w (collections, the video gallery, games) get it now for every kind.

Numbers

Demo media (D:\WittyDemoMedia) scanned by scripts/perf (#89's benchmark), same data folder, base build then this one, Windows, busy machine:

Making a copy took 0.15 to 1.3 s the first time on that busy machine, then it's a file read (15 to 120 ms there).

How it was tested

• New TestPosterAskedSmallerIsSmaller (internal/server): a 1000×1500 film poster asked ?w=300 comes back as a 320×480 JPEG (410 KiB → 47 KiB for the noisy test picture), without w as the original, and a poster outside the library's folders stays 404 both ways.
• GOWORK=off go test ./internal/library -run 'PosterThumb|Trim|Cache' and the new server test on Windows: ok. go vet on both packages.
• Not yet: the whole internal/server run (CI does it; it takes about ten minutes).

Not tried

• On a real phone or TV: the web app doesn't ask for small posters yet (Web app performance: smaller, faster pages on slow devices #90).

Changelog line

Posters asked for at a smaller size are made smaller for films, shows, music and books too, not only games, so lists load much less (one demo poster went from 1.6 MB to 25 KB).

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