feat(api): add GET/PUT /api/me/listenbrainz endpoints

Exposes user ListenBrainz config (token_set bool, enabled, last_scrobbled_at)
via write-only token semantics; enforces no-enable-without-token at the API layer.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/api/api.go

@@ -29,6 +29,8 @@ func Mount(r chi.Router, pool *pgxpool.Pool, logger *slog.Logger, events *playev
 			authed.Use(auth.RequireUser(pool))
 			authed.Post("/auth/logout", h.handleLogout)
 			authed.Get("/me", h.handleGetMe)
+			authed.Get("/me/listenbrainz", h.handleGetListenBrainz)
+			authed.Put("/me/listenbrainz", h.handlePutListenBrainz)
 
 			authed.Get("/artists", h.handleListArtists)
 			authed.Get("/artists/{id}", h.handleGetArtist)