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/i18n/locales/en/downloads.json

@@ -71,6 +71,12 @@
     "items": "{{used}} / {{max}} items",
     "unlimited": "Unlimited"
   },
+  "source": {
+    "open": "Open the original page in a new tab",
+    "copy": "Copy source link",
+    "copied": "Source link copied to clipboard",
+    "copyFailed": "Couldn't copy the link"
+  },
   "rename": {
     "title": "Rename download",
     "label": "Display name",