feat(downloads): store + show a canonical "downloaded from" source URL

Every download now records the clean source page URL and shows it on the
Downloads page (open in a new tab, or copy to clipboard). The worker stores
yt-dlp's canonical webpage_url on the asset (migration 0043 adds
media_assets.source_webpage_url); the serializer prefers it and falls back to a
URL derived from source_kind+source_ref, so YouTube, external YouTube links and
external URLs (e.g. Facebook reels) all get a correct reference, and queued/older
rows work before the worker fills it. Edit clips return null (a clip's source is
the user's own earlier download, not a web page). i18n en/hu/de.

frontend/src/components/DownloadCenter.tsx

@@ -3,7 +3,9 @@ import { useTranslation } from "react-i18next";
 import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
 import {
   Ban,
+  Copy,
   Download,
+  Link2,
   Pause,
   Play,
   Plus,
@@ -101,6 +103,51 @@ function IconBtn({
   );
 }
 
+// Compact display of a URL: drop the protocol + trailing slash so "youtube.com/watch?v=…" fits.
+function displayUrl(url: string): string {
+  try {
+    const u = new URL(url);
+    return (u.host + u.pathname + u.search).replace(/\/$/, "");
+  } catch {
+    return url;
+  }
+}
+
+// "Downloaded from" reference: opens the original page in a new tab, or copies the link.
+function SourceRef({ url }: { url: string }) {
+  const { t } = useTranslation();
+  const copy = async () => {
+    try {
+      await navigator.clipboard.writeText(url);
+      notify({ level: "success", message: t("downloads.source.copied") });
+    } catch {
+      notify({ level: "error", message: t("downloads.source.copyFailed") });
+    }
+  };
+  return (
+    <div className="flex items-center gap-1 mt-1 text-xs text-muted min-w-0">
+      <Link2 className="w-3 h-3 shrink-0" />
+      <a
+        href={url}
+        target="_blank"
+        rel="noopener noreferrer"
+        title={`${t("downloads.source.open")} — ${url}`}
+        className="truncate hover:text-accent hover:underline"
+      >
+        {displayUrl(url)}
+      </a>
+      <button
+        onClick={copy}
+        aria-label={t("downloads.source.copy")}
+        title={t("downloads.source.copy")}
+        className="shrink-0 p-0.5 rounded hover:bg-surface hover:text-fg transition"
+      >
+        <Copy className="w-3 h-3" />
+      </button>
+    </div>
+  );
+}
+
 function DownloadRow({
   job,
   children,
@@ -131,6 +178,7 @@ function DownloadRow({
           {job.asset?.duration_s ? <span className="text-muted">· {formatDuration(job.asset.duration_s)}</span> : null}
           {job.error ? <span className="text-red-400 truncate">· {job.error}</span> : null}
         </div>
+        {job.source_url && <SourceRef url={job.source_url} />}
         {running && (
           <div className="mt-1.5">
             {job.phase && !DOWNLOAD_PHASES.includes(job.phase) ? (