feat(scrobble): Head-to-Head "Musik-Match" — compare two users' listening

Adds GET /scrobble/users (distinct users for the partner picker) and
GET /scrobble/compare?with= to the scrobble service. Taste match = cosine
of the two users' artist-play vectors (0..100); also returns shared artists
(mirrored bars), shared tracks, and each side's unique top artists. Reuses
the existing per-user play-log — no new data capture.

Web UI src/ui/compare.js (overlay from Settings "🎧 Musik-Match"): partner
picker fed by /scrobble/users, match-score ring, per-user totals, shared +
unique lists with clickable artist/album rows. Cosine + shared logic verified
against a hand computation on an in-memory DB. Android pending (parity: 🔜).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

CHANGELOG.md

@@ -57,6 +57,7 @@ Versioning: two-part `MAJOR.MINOR` — each release bumps the minor (features an
 | Timestamped play-log (own scrobble)  | ✅        | ✅      |
 | Listening statistics view            | ✅        | ✅      |
 | Stats cover art (artist/album)       | ✅        | ✅      |
+| Musik-Match (Head-to-Head)           | ✅        | 🔜      |
 | Yearly Wrapped                       | 🔜        | 🔜      |
 | Romaji / Japanese sort           | ✅        | ❌      |
 | Settings (Navidrome, ABS, etc.)  | ✅        | ✅      |