remotes: flush cached metadata when a remote's base_url changes (#120)

## Why

Switching a remote's backend (`base_url`) left artifactapi serving the previously-cached mutable metadata (repodata / Release / APKINDEX) until TTL expiry, so requests could keep pointing at the old upstream. `cache.FlushRemote` already existed but was wired to nothing.

## How

- Inject a `MetadataFlusher` (satisfied by `*cache.Redis`) into `RemotesHandler` via `NewRemotesHandler`; `server.go` passes `s.cache`.
- On update, read the existing remote first, then after a successful DB update flush the remote's cached metadata when `base_url` changed, so the next request re-fetches fresh from the new upstream.
- Keep scope to `base_url` (upstream identity); a flush failure is logged as a warning and does not fail the request since the DB update already landed.
- Add tests: a `base_url` change flushes exactly once, an unchanged `base_url` does not flush, and a flush error still returns 200.

Reviewed-on: #120
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

internal/server/server.go

@@ -175,7 +175,7 @@ func (s *Server) routes() chi.Router {
 	r.Mount("/api/v1", proxyHandler.Routes())
 	r.Mount("/v2", proxyHandler.DockerV2Routes())
 
-	remotesHandler := v2.NewRemotesHandler(s.db, map[models.PackageType]v2.Primer{
+	remotesHandler := v2.NewRemotesHandler(s.db, s.cache, map[models.PackageType]v2.Primer{
 		models.PackageGitHubRPM:    s.syncer,
 		models.PackageGitHubDeb:    s.debSyncer,
 		models.PackageGitHubAlpine: s.alpineSyncer,