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/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)
 	}