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/frontend.go

@@ -849,6 +849,54 @@ func registerFrontendRoutes(cfg *Config) {
 		return c.HTML(http.StatusOK, buf.String())
 	}))
 
+	frontendProtected.GET("/admin/archived", handlers.AdminMiddleware(func(c *echo.Context) error {
+		user, err := getTemplateUserWithTheme(c, cfg)
+		if err != nil {
+			return renderErrorPage(c, "Error loading user", "user_load_error")
+		}
+
+		rows, err := cfg.Queries.ListHiddenMediaItems(c.Request().Context())
+		if err != nil {
+			log.Printf("ListHiddenMediaItems failed: %v", err)
+			rows = []database.ListHiddenMediaItemsRow{}
+		}
+
+		retentionDays := config.ArchiveRetentionDays()
+		items := make([]templates.ArchivedItem, 0, len(rows))
+		for _, row := range rows {
+			itemUUID, _ := uuid.FromBytes(row.ID.Bytes[0:16])
+			item := templates.ArchivedItem{
+				ID:           itemUUID.String(),
+				Title:        row.Title,
+				Author:       getText(row.Author),
+				LibraryName:  row.LibraryName,
+				FilePath:     row.FilePath,
+				MissingScans: row.MissingScanCount,
+			}
+			if row.ArchivedAt.Valid {
+				archived := row.ArchivedAt.Time
+				item.ArchivedAt = &archived
+				switch {
+				case retentionDays <= 0:
+					item.Status = "Archived - kept until manually purged (retention disabled)"
+				default:
+					purge := archived.AddDate(0, 0, retentionDays)
+					item.PurgeAt = &purge
+					item.Status = "Archived - permanently deleted " + purge.Format("Jan 2, 2006")
+				}
+			} else {
+				item.Status = fmt.Sprintf("Missing from disk - archives on the next scan (%d/2)", row.MissingScanCount)
+			}
+			items = append(items, item)
+		}
+
+		var buf bytes.Buffer
+		if err := templates.AdminArchived(user, items, retentionDays).Render(c.Request().Context(), &buf); err != nil {
+			return err
+		}
+		return c.HTML(http.StatusOK, buf.String())
+	}))
+
 	frontendProtected.GET("/admin/library", handlers.AdminMiddleware(func(c *echo.Context) error {
 		user, err := getTemplateUserWithTheme(c, cfg)
 		if err != nil {