feat(admin): archived-items page with purge dates, restore, and purge

Admins need to see what the archive lifecycle is holding: a dedicated
/admin/archived page listing every hidden item (archived or still in
the missing grace window) with library, file path, status, and - when
retention is enabled - the exact date it will be permanently deleted
(archived_at + ARCHIVE_RETENTION_DAYS).

Per row: Restore (POST /api/media-items/:id/unarchive, clears the
archive state so it reappears; if the file is still gone the next scan
hides it again) and Delete (existing DELETE endpoint for single-row
purge with its reading history). Purge All Archived reuses the existing
bulk button. The library admin banner links to the page and keeps its
purge button; row actions use data attributes with delegated listeners
in admin.ts since this templ version has no JSFunctionCall helper.

Verified live: page 200 with purge dates shown, unarchive returned 204
and reset the row, per-item delete and bulk purge ({purged:1}) both
removed their rows with no leftovers.

internal/router/media.go

@@ -63,6 +63,7 @@ func registerMediaRoutes(cfg *Config) {
 	admin.POST("/media-items/:id/rescan", cfg.MediaHandler.RescanMediaItem)
 	admin.DELETE("/media-items/:id", cfg.MediaHandler.DeleteMediaItem)
 	admin.POST("/media-items/purge-archived", cfg.MediaHandler.PurgeArchivedMediaItems)
+	admin.POST("/media-items/:id/unarchive", cfg.MediaHandler.UnarchiveMediaItem)
 
 	// Shelf management (protected)
 	protected.POST("/devices/:id/shelves", cfg.MediaHandler.AddToShelf)