feat(admin): purge-archived endpoint with an archived-items banner

Bulk escape hatch for archived rows (files missing from disk for 2+
scans) so a mass external deletion never has to wait out the retention
window or be clicked away row by row:

- POST /api/media-items/purge-archived (admin only) hard-deletes all
  archived items and returns the purged count; reading history goes with
  the rows, so the call is confirmed in the UI first.
- The library admin page shows an 'Archived items: N' card (only when
  non-zero) with a Purge Archived Now button that calls the endpoint,
  toasts the result, and reloads.
- Frontend admin JS exposes window.purgeArchivedItems following the
  existing localStorage-bearer-token pattern.

internal/router/frontend.go

@@ -884,7 +884,8 @@ func registerFrontendRoutes(cfg *Config) {
 		}
 
 		var buf bytes.Buffer
-		err = templates.AdminLibrary(user, libData, userData).Render(c.Request().Context(), &buf)
+		archivedCount, _ := cfg.Queries.CountArchivedMediaItems(c.Request().Context())
+		err = templates.AdminLibrary(user, libData, userData, archivedCount).Render(c.Request().Context(), &buf)
 		if err != nil {
 			return err
 		}