* Separate artifact and metadata cache metrics on /metrics and the analytics page
* Serve metadata cache stats from a shared snapshot and split Grafana cache panels by kind
* 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>
A response arriving during a hold was sent before it began, so a late
success no longer releases the host; only one after the hold has ended
does.
Artifact fetches check RateLimitGate.Held before calling the fetcher, so
held cache misses fail with the HeldError without counting as failures
in the host's circuit breaker.
Every handler that answers an upstream fetch error with 502 now answers
a held request with 503 and Retry-After instead.
OnHold and RecordUpstreamRateLimitHold drop the unused duration.
