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/lib/api.ts

@@ -698,6 +698,7 @@ export interface DownloadJob {
   created_at: string | null;
   source_kind: string;
   source_ref: string;
+  source_url: string | null; // clean "downloaded from" URL (null for edit clips)
   profile_id: number | null;
   spec: DownloadSpec;
   display_name: string | null;