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/db/queries/users.sql

@@ -37,7 +37,7 @@ WHERE id = $1;
 SELECT
     u.listenbrainz_token,
     u.listenbrainz_enabled,
-    (SELECT MAX(pe.scrobbled_at)
+    (SELECT MAX(pe.scrobbled_at)::timestamptz
      FROM play_events pe
      WHERE pe.user_id = u.id) AS last_scrobbled_at
 FROM users u