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.

backend/alembic/versions/0049_download_display_meta.py

@@ -0,0 +1,38 @@
+"""per-download display metadata + auto channel URL
+
+Revision ID: 0049_download_display_meta
+Revises: 0048_plex_playlists
+Create Date: 2026-07-07
+
+Lets a user override a download's channel and attach extra reference links (beyond the source),
+and captures yt-dlp's canonical channel/uploader URL so an auto-filled channel becomes clickable.
+
+- media_assets.uploader_url: the channel page URL yt-dlp recorded (nullable; filled by the worker
+  on extraction — older rows just stay plain text).
+- download_jobs.display_uploader / display_uploader_url: per-user channel-name + channel-link
+  overrides (like display_name — display metadata only, never touches the on-disk file).
+- download_jobs.extra_links: optional list of extra reference URLs shown on the card + watch page.
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0049_download_display_meta"
+down_revision: Union[str, None] = "0048_plex_playlists"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column("media_assets", sa.Column("uploader_url", sa.Text(), nullable=True))
+    op.add_column("download_jobs", sa.Column("display_uploader", sa.String(length=255), nullable=True))
+    op.add_column("download_jobs", sa.Column("display_uploader_url", sa.Text(), nullable=True))
+    op.add_column("download_jobs", sa.Column("extra_links", sa.JSON(), nullable=True))
+
+
+def downgrade() -> None:
+    op.drop_column("download_jobs", "extra_links")
+    op.drop_column("download_jobs", "display_uploader_url")
+    op.drop_column("download_jobs", "display_uploader")
+    op.drop_column("media_assets", "uploader_url")