`git.listLanguages` has been one of the method that fails most often. Opening multiple git repos simultaneously can eaily cause OOM and language indexing itself usually takes super long. So several changes: - use `gitea.CatFileBatch` instead of go-git to avoid OOM - skip files larger than 16KB - sync HEAD ref language stats in knotmirror db - cache language stats info by commits (30d TTL) When syncing HEAD ref language stats, we do indexing on background. KnotMirror maintains internal "repo_stats_update" queue and right after `doResync` is done, enqueue the language stat indexing job so we can pre-index the language stats of HEAD ref. It's ok to spam this queue because all later events will be eventually ignored as we are resolving HEAD lazily. Signed-off-by: Seongmin Lee <git@boltless.me>