The edit (pencil) action now edits a download's full display metadata — title,
channel name, channel link and any number of extra reference URLs — instead of
just the name. The channel and links render as clickable links on the library
card, and the channel link is auto-filled from the source (yt-dlp channel_url)
when available. Shared watch pages resolve the same per-download overrides, so a
rename/channel/link edit is reflected on the public /watch page too, with every
link clickable.
Adds migration 0049 (media_assets.uploader_url; download_jobs.display_uploader,
display_uploader_url, extra_links) and generalizes the rename endpoint into a
metadata update with URL validation. EN/HU/DE strings included.
