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>

pkg/models/remote.go

@@ -84,6 +84,10 @@ type Remote struct {
 	ReleasesRemote string `json:"releases_remote,omitempty"`
 	ManagedBy      string `json:"managed_by,omitempty"`
 
+	// CacheRepodata keeps a github_rpm remote's generated repodata in memory,
+	// rebuilding it only when its metadata rows change.
+	CacheRepodata bool `json:"cache_repodata"`
+
 	CreatedAt time.Time `json:"created_at"`
 	UpdatedAt time.Time `json:"updated_at"`
 }