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.

web/src/admin.ts

@@ -249,6 +249,46 @@ function stopScanStatusPolling(): void {
   }
 }
 
+// Purge all archived items (files missing from disk for 2+ scans). Exposed on
+// window for the library admin page's inline button; reading history is
+// deleted with the rows, so a confirm dialog guards it.
+async function purgeArchivedItems(): Promise<void> {
+  const token = localStorage.getItem("token");
+  if (!token) return;
+
+  if (
+    !window.confirm(
+      "Permanently delete all archived items? Their reading progress, notes, and highlights will be lost.",
+    )
+  ) {
+    return;
+  }
+
+  try {
+    const resp = await fetch("/api/media-items/purge-archived", {
+      method: "POST",
+      headers: { Authorization: `Bearer ${token}` },
+    });
+    if (!resp.ok) {
+      const err = await resp.json().catch(() => ({}));
+      throw new Error(err.error || "Failed to purge archived items");
+    }
+    const data = await resp.json();
+    showToast(
+      `Purged ${data.purged} archived item${data.purged === 1 ? "" : "s"}`,
+      "success",
+    );
+    setTimeout(() => window.location.reload(), 700);
+  } catch (e) {
+    showToast(
+      e instanceof Error ? e.message : "Failed to purge archived items",
+      "error",
+    );
+  }
+}
+
+(window as any).purgeArchivedItems = purgeArchivedItems;
+
 export {
   hideScanProgress,
   loadWatchStatus,