remotes: flush cached metadata when a remote's base_url changes

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

- Inject a MetadataFlusher (satisfied by *cache.Redis) into RemotesHandler.
- 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.
- A flush failure is logged as a warning and does not fail the request; the
  DB update already landed.
- Add tests: base_url change flushes exactly once, an unchanged base_url
  does not flush, and a flush error still returns 200.

internal/api/v2/errorpaths_test.go

@@ -57,7 +57,7 @@ func do(t *testing.T, h http.Handler, method, path, body string) int {
 }
 
 func TestRemotesErrorPaths(t *testing.T) {
-	h := NewRemotesHandler(closedDB(t), nil).Routes()
+	h := NewRemotesHandler(closedDB(t), nil, nil).Routes()
 	if c := do(t, h, "GET", "/", ""); c != 500 {
 		t.Errorf("list with dead db = %d, want 500", c)
 	}