fix cloud library scale and playback compatibility

Root cause: display-only cloud library filtering was reused by scan jobs, merged cloud mounts could be skipped, and OpenList listing relied on WebDAV/first-page behavior that could cap huge directories around 100 items. Cloud scans also let new items consume probe budget before existing rows with missing track/artwork metadata.\n\nChanges:\n- split display filtering from scannable cloud filtering so merged cloud mounts still scan\n- add OpenList API pagination with WebDAV fallback\n- prioritize existing cloud media missing metadata before new imports\n- restrict automatic cloud sync to one successful 19:00-21:00 daily window and keep manual scan immediate\n- disable startup cloud full-scan by default, with an explicit opt-in setting\n- improve Emby/cloud playback compatibility and cache/search/test coverage from the continued work

internal/service/stats.go

@@ -62,7 +62,7 @@ func (s *StatsService) Compute(ctx context.Context, dataDir string) (*Snapshot,
 	if err != nil {
 		return nil, err
 	}
-	libs = FilterShadowedCloudLibraries(libs)
+	libs = FilterDisplayCloudLibraries(ctx, s.repo, libs)
 	activeLibraryIDs := make([]string, 0, len(libs))
 	for _, lib := range libs {
 		if !lib.Enabled {