feat(downloads): editable details with clickable channel and extra links

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.

frontend/src/i18n/locales/en/downloads.json

@@ -56,6 +56,7 @@
     "retry": "Retry",
     "download": "Save to my device",
     "rename": "Rename",
+    "editDetails": "Edit details",
     "share": "Share",
     "edit": "Edit / trim",
     "removeShared": "Remove from my list"
@@ -83,6 +84,17 @@
     "hint": "Only changes the name you see and download with — the stored file keeps its own name.",
     "save": "Save"
   },
+  "edit": {
+    "title": "Edit details",
+    "name": "Title",
+    "channel": "Channel",
+    "channelUrl": "Channel link",
+    "extraLinks": "Extra links",
+    "addLink": "Add link",
+    "removeLink": "Remove link",
+    "hint": "Display only — the stored file keeps its own name. The channel and links also show on the card and on shared watch pages.",
+    "save": "Save"
+  },
   "share": {
     "title": "Share download",
     "label": "Recipient email",