Evict remote objects from every cache layer (#128)

Evicting a remote object only deleted its Postgres row. The S3 index and the Redis TTL/ETag keys survived, so stale mirror metadata such as EPEL repodata kept being served.

- evict the artifact row, S3 index and Redis keys via `Engine.Evict`
- evict a directory with `<dir>/*`; other wildcards return 400, unknown remotes 404
- wait on the per-path fetch lock for single-path evicts; return 503 on timeout or a Redis error
- wildcard evicts take no lock; a fetch already in flight may re-cache its path
- return S3 list errors from `DeletePrefix` instead of deleting nothing

Reviewed-on: #128
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

internal/database/artifacts.go

@@ -103,6 +103,13 @@ func (db *DB) DeleteArtifact(ctx context.Context, remoteName, path string) error
 	return err
 }
 
+// DeleteArtifactsByPrefix removes every artifact row of a remote whose path
+// starts with prefix.
+func (db *DB) DeleteArtifactsByPrefix(ctx context.Context, remoteName, prefix string) error {
+	_, err := db.Pool.Exec(ctx, `DELETE FROM artifacts WHERE remote_name = $1 AND left(path, length($2)) = $2`, remoteName, prefix)
+	return err
+}
+
 func (db *DB) InsertAccessLog(ctx context.Context, remoteName, path string, cacheHit bool, sizeBytes int64, upstreamMS int, clientIP string) error {
 	_, err := db.Pool.Exec(ctx, `
 		INSERT INTO access_log (remote_name, path, cache_hit, size_bytes, upstream_ms, client_ip)