Main stopped compiling: the v21 column drop treated cached_at as
never-read, but the fresh-cache-first playback metadata gate
(ApiCacheSingleton.getIfFresh, from the playback start-latency fix)
reads it on every Plex and Jellyfin playback start, and a later
cleanup removed the put() stamp the gate depends on.
Reinstate the column, keep the v21 migration to the connections
half only (it never shipped in a release), restore the explicit
put() timestamp - on conflict the upsert only updates the
companion's columns, so without it a refreshed row would keep its
original write time and read as permanently stale - and copy
cached_at through the pinned-metadata rescope statement again. The
v21 migration test now pins that api_cache is left untouched.
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