A streamed cache hit whose bytes don't match the row's digest now
records an integrity failure and deletes the row, queueing its blob for
deletion, as a corrupt cached artifact is handled. The next request
fetches the document from upstream instead of aborting until the TTL
runs out.
* max_size now counts cached metadata as well as artifacts, and eviction drops whichever was used least recently, so metadata no longer piles up for good (#461). Metadata rows record when they were last served, an artifact not yet served counts from its download, and Arch stores a database blob again if eviction removed it under a refreshed row.
* Shorter comments on the metadata last-access migration.
* Show evicted metadata on the analytics page next to evicted artifacts, and cover the new counter in the metric surface test.
* Evicting metadata now drops the row and queues its path in one transaction, so a crash before the blob is deleted leaves it to reclaim instead of orphaning it.
* The last-access backfill now runs in the background in batches of 1000 instead of inside the migration, resuming after a restart, and until a row is backfilled eviction orders it by its write time.
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Co-authored-by: Andrew Nesbitt <andrewnez@gmail.com>
