feat(subsonic): add /rest/star and /rest/unstar handlers

Validate-all-first atomicity on star: a missing entity refuses the whole
call with Subsonic error 70. Unstar is a best-effort delete (missing
entities are no-ops, matching client expectations after library moves).

internal/subsonic/subsonic.go

@@ -38,6 +38,8 @@ func Mount(r chi.Router, pool *pgxpool.Pool, logger *slog.Logger, cfg Config, ev
 		register(sub, "/download", m.handleDownload)
 		register(sub, "/getCoverArt", m.handleGetCoverArt)
 		register(sub, "/scrobble", m.handleScrobble)
+		register(sub, "/star", m.handleStar)
+		register(sub, "/unstar", m.handleUnstar)
 		register(sub, "/getUser", handleGetUser)
 
 		// Subsonic clients expect every /rest/* miss to come back as a