fix(db): drop redundant quarantine user index + tighten ListQuarantineForUser

- Composite PK (user_id, track_id) already serves WHERE user_id queries;
  the secondary (user_id, created_at DESC) index just amplified writes.
- ListQuarantineForUser now selects only the joined fields the SPA card
  actually renders (~10 columns) rather than embedding three full structs
  (~35 columns); halves wire/DB bandwidth before consumers exist.
- max(q.created_at)::timestamptz cast emits pgtype.Timestamptz instead of
  interface{} so handlers can read latest_at without a type-assert.

internal/db/migrations/0011_lidarr_quarantine.down.sql

@@ -2,7 +2,6 @@ DROP INDEX IF EXISTS lidarr_quarantine_actions_created_idx;
 DROP INDEX IF EXISTS lidarr_quarantine_actions_track_idx;
 DROP TABLE IF EXISTS lidarr_quarantine_actions;
 DROP TYPE  IF EXISTS lidarr_quarantine_action_kind;
-DROP INDEX IF EXISTS lidarr_quarantine_user_idx;
 DROP INDEX IF EXISTS lidarr_quarantine_track_idx;
 DROP TABLE IF EXISTS lidarr_quarantine;
 DROP TYPE  IF EXISTS lidarr_quarantine_reason;