feat: add size-based cache eviction (#1170)

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>

internal/app/cmd/cache-server.go

@@ -10,6 +10,7 @@ import (
 	"os/signal"
 
 	"gitea.com/gitea/runner/act/artifactcache"
+	"gitea.com/gitea/runner/internal/app/run"
 	"gitea.com/gitea/runner/internal/pkg/config"
 
 	log "github.com/sirupsen/logrus"
@@ -52,13 +53,14 @@ func runCacheServer(configFile *string, cacheArgs *cacheServerArgs) func(cmd *co
 		if secret == "" {
 			return errors.New("cache.external_secret (or cache.external_secret_file) must be set for cache-server; configure the same value on each runner that points at this server via cache.external_server")
 		}
-		cacheHandler, err := artifactcache.StartHandler(
-			dir,
-			host,
-			port,
-			secret,
-			log.StandardLogger().WithField("module", "cache_request"),
-		)
+		cacheHandler, err := artifactcache.StartHandler(artifactcache.Options{
+			Dir:            dir,
+			OutboundIP:     host,
+			Port:           port,
+			InternalSecret: secret,
+			Policy:         run.CachePolicy(cfg),
+			Logger:         log.StandardLogger().WithField("module", "cache_request"),
+		})
 		if err != nil {
 			return err
 		}