Replace the per-tracker json_serializable session classes and account stores with a shared TrackerSession (service-aware persisted validation), a single TrackerAccountStore, a shared TrackerHttpClient, and an AnimeListTrackerBase mixin for MAL/AniList. Scope the per-service rebind guard so a disconnect can't abort a racing profile load. Add migration-safety, MAL refresh, episode-count cache, and content-type tests.