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.

backend/app/worker.py

@@ -221,6 +221,7 @@ def _fill_asset_meta_early(asset_id: int, info: dict) -> None:
         asset.thumbnail_url = info.get("thumbnail")
         asset.duration_s = int(info["duration"]) if info.get("duration") else None
         asset.upload_date = _parse_upload_date(info.get("upload_date"))
+        asset.source_webpage_url = info.get("webpage_url")
         db.commit()
 
 
@@ -409,6 +410,7 @@ def _download(job_id: int, asset_id: int, source_kind: str, source_ref: str, spe
                 asset.uploader = meta.uploader
                 asset.upload_date = meta.upload_date
                 asset.thumbnail_url = meta.thumbnail_url
+                asset.source_webpage_url = info.get("webpage_url")
                 asset.nfo_written = nfo_ok
                 asset.error = None
                 asset.last_access_at = datetime.now(timezone.utc)