feat(hub): downsample external cover art and dedupe cache variants (#145)

[benders]

Closes #145.

Problem

The art cache filled its 100 MB cap almost immediately. Two causes (see investigation on the issue):

1. The external-art branch of getCoverArt (fanart.tv / Last.fm) ignored the size param but keyed the cache by it — identical full-resolution ~1000×1000 bytes (100–500 KB) were cached once per requested size. The SPA requests five variants (48/80/300/400/unsized), so one cover could occupy ~1.5 MB across five slots.
2. Unsized requests cached/served original bytes — hi-res embedded art can be several MB per image.

Changes

• Effective size normalization (hub/src/routes/subsonic/art-size.ts): every request resolves to a clamped size in [16, 1024]; absent/invalid defaults to 1024. Cache key is always ${id}:${effSize}, so unsized and size=1024 dedupe; old-format entries LRU-evict naturally (no migration).
• Hub-side downsampling of external art (hub/src/services/art-resize.ts, new sharp dep): the external branch resizes fetched bytes to fit the effective size before caching and transmitting — a 48px PlayerBar thumbnail no longer downloads 300 KB. Local/peer art is untouched (already resized upstream by Navidrome, which now always receives the effective size). GIF/SVG pass through unresized; sharp failure on corrupt bytes falls back to the original buffer.
• Default cache cap 100 MB → 1 GB — even downsampled, one 300px variant across a merged library is hundreds of MB; 100 MB churned as a revolving door. Still overridable via ART_CACHE_MAX_BYTES env and the admin cache API.
• Docs: hub-internals.md (env table, art-cache section), opensubsonic.md (getCoverArt size cap caveat).

Behavior change note

size values above 1024 are now clamped and unsized requests return at most 1024px (this caps the #200 lightbox at 1024 — deemed sufficient for a music player UI).

Testing

• pnpm verify green: hub 799 tests / frontend 145, typecheck clean, boundary lint clean.
• pnpm lint exits zero with no warnings.
• pnpm test:federation green (full py-sonic compat + eviction/re-admission cluster suite).
• New coverage: effectiveArtSize unit cases; external resize asserted via sharp().metadata() on the response body; dedupe via x-cache: HIT + single upstream fetch; 4096→1024 clamp; corrupt-bytes fallback; GIF passthrough.

🤖 Generated with Claude Code

https://claude.ai/code/session_01SwwPcrohddTUN7kmQVedQC