Add Jellyfin NFO sidecar generation for downloaded videos

Writes a Kodi/Jellyfin-compatible .nfo XML file next to each .mp4 on
download completion, deletes it when the download record is removed, and
exposes POST /api/downloads/nfo/generate to backfill NFOs for existing
downloads. Frontend adds a "Generate NFO" button in the Downloads header.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/pages/Downloads.jsx

@@ -1,7 +1,7 @@
 import { useState, useMemo } from "react";
 import { useNavigate } from "react-router-dom";
 import { useQuery, useMutation, useQueryClient } from "@tanstack/react-query";
-import { getDownloads, deleteDownload, deleteAllDownloads, restoreDownload, getActiveTasks } from "../api";
+import { getDownloads, deleteDownload, deleteAllDownloads, restoreDownload, getActiveTasks, generateNfoFiles } from "../api";
 import SortPicker from "../components/SortPicker";
 
 const HISTORY_SORTS = [
@@ -80,6 +80,12 @@ export default function DownloadsPage() {
     },
   });
 
+  const [nfoResult, setNfoResult] = useState(null);
+  const nfoMut = useMutation({
+    mutationFn: generateNfoFiles,
+    onSuccess: (r) => setNfoResult(r.data.generated),
+  });
+
   if (isLoading) {
     return (
       <div className="flex items-center justify-center py-16">
@@ -104,6 +110,15 @@ export default function DownloadsPage() {
     <div className="flex flex-col gap-6">
       <div className="flex items-center justify-between">
         <h1 className="font-display font-bold text-2xl text-zinc-100">Downloads</h1>
+        <div className="flex items-center gap-3">
+          <button
+            onClick={() => { setNfoResult(null); nfoMut.mutate(); }}
+            disabled={nfoMut.isPending}
+            title="Generate Jellyfin .nfo sidecar files for all completed downloads"
+            className="text-sm text-zinc-500 hover:text-zinc-300 disabled:opacity-50 transition-colors"
+          >
+            {nfoMut.isPending ? "Generating…" : nfoResult != null ? `Generated ${nfoResult} NFO` : "Generate NFO"}
+          </button>
         {hasRemovable && (
           confirmClear ? (
             <div className="flex items-center gap-2">
@@ -131,6 +146,7 @@ export default function DownloadsPage() {
             </button>
           )
         )}
+        </div>
       </div>
 
       {activeTasks.length > 0 && (