Add cache_repodata toggle for github_rpm remotes (#168)

github_rpm remotes regenerate repomd and all three gz files from every metadata row on each request, which is expensive for large repos under repeated dnf traffic.

- add a per-remote `cache_repodata` toggle (column, model, v2 API, docs)
- cache built repodata per replica, rebuilt via singleflight when the `count(*)`+`max(id)` row fingerprint moves
- store a build only if no newer build has landed; evict idle entries hourly
- map cached-path DB errors like the uncached path: 503 + Retry-After on cancel/deadline, generic 500 otherwise
- check `cacheEmpty` via the fingerprint for all github_rpm remotes, not only cached ones

Reviewed-on: #168
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

migrations/0010_remote_cache_repodata.sql

@@ -0,0 +1 @@
+ALTER TABLE remotes ADD COLUMN IF NOT EXISTS cache_repodata BOOLEAN NOT NULL DEFAULT FALSE;