feat(downloads): clickable channels + poster fallback for thumbnail-less sources

Two visual gaps for non-catalog downloads:
- Channel link: YouTube already exposes channel_url; Facebook exposes none but a
  numeric uploader_id that resolves at facebook.com/<id>. `_uploader_url` derives
  it so the auto-detected channel renders as a real clickable link.
- Poster: a source with no thumbnail (e.g. a direct reddit HLS URL) showed a
  blank image box. The worker now cuts a representative frame with ffmpeg
  (`ensure_poster`) into the `<base>.jpg` sidecar and records `poster_path`
  (migration 0050). The card, the public watch page (<video poster> + og:image),
  and link previews fall back to it via new authed + public poster endpoints.

Adds `app.downloads.backfill` (one-off, re-run-safe) to fill uploader_url
(re-extract YouTube/Facebook metadata) and posters for pre-existing downloads.

backend/alembic/versions/0050_asset_poster.py

@@ -0,0 +1,28 @@
+"""local poster path for thumbnail-less downloads
+
+Revision ID: 0050_asset_poster
+Revises: 0049_download_display_meta
+Create Date: 2026-07-07
+
+Some sources (a direct media URL like a reddit v.redd.it HLS manifest) carry no thumbnail, so the
+download shows a blank image box. The worker now extracts a representative frame with ffmpeg and
+saves it as a poster sidecar; `poster_path` (relative to download_root) records it so the card /
+watch page / link preview can fall back to it. Nullable — only set when there was no thumbnail.
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0050_asset_poster"
+down_revision: Union[str, None] = "0049_download_display_meta"
+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("poster_path", sa.Text(), nullable=True))
+
+
+def downgrade() -> None:
+    op.drop_column("media_assets", "poster_path")