The cache server retired entries 30 days after creation regardless of use, so a job that ran often enough to keep its cache warm still lost it on a fixed schedule. Nothing bounded the disk either.
Retention now counts from last access alone, and a repository over its limit sheds least recently accessed entries until it fits, enforced on commit as well as on the periodic sweep.
```yaml
cache:
retention: 168h # remove entries not accessed for seven days
repo_size_limit: 10GB # cap each repository
size_limit: 0 # cap the whole cache, off by default
sweep_interval: 1h # minimum time between sweeps
```
Sizes accept `10GB`, `512mb`, `1TiB` or a plain byte count, binary either way. Leave a key out for its default; `0` turns a limit off, and `0s` does the same for `retention`. Whatever these allow, the cache also sheds entries to keep free space above `health_check.min_free_disk_space_mb` when health checks are enabled, so it cannot grow past the point where the runner stops accepting work.
Supporting fixes: serving an entry stamps its access time, so a find cannot hand a job a download URL for an entry the next eviction is about to remove; an entry larger than the limit is dropped on its own account rather than emptying its repository to make room; and a blob that cannot be unlinked keeps its row, so the next sweep retries instead of orphaning bytes no limit can account for.
Closes https://gitea.com/gitea/runner/issues/1168
---------
Co-authored-by: silverwind <me@silverwind.io>
Reviewed-on: https://gitea.com/gitea/runner/pulls/1170
Reviewed-by: silverwind <2021+silverwind@noreply.gitea.com>
