Fix channel avatar: include channel_thumbnail_url in video response

Reading from the channels query cache was unreliable (cache might not be
loaded, or channel not followed). Add c.thumbnail_url AS channel_thumbnail_url
to _VIDEO_SELECT so every video response carries its channel avatar directly.
VideoCard uses it with cache as fallback.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/routers/videos.py

@@ -68,6 +68,7 @@ class VideoDetail(BaseModel):
     view_count: Optional[int] = None
     like_count: Optional[int] = None
     dislike_count: Optional[int] = None
+    channel_thumbnail_url: Optional[str] = None
 
     model_config = {"from_attributes": True}
 
@@ -426,6 +427,7 @@ _VIDEO_SELECT = """
     SELECT v.id, v.youtube_video_id, v.title, v.description, v.thumbnail_url,
            v.duration_seconds, v.published_at, v.tags, v.category, v.view_count, v.like_count, v.dislike_count,
            c.id AS channel_id, c.name AS channel_name, c.youtube_channel_id AS channel_youtube_id,
+           c.thumbnail_url AS channel_thumbnail_url,
            COALESCE(uv.watched, 0) AS watched,
            COALESCE(uv.watch_progress_seconds, 0) AS watch_progress_seconds,
            COALESCE(uv.downloaded, 0) AS is_downloaded,

frontend/src/components/VideoCard.jsx

@@ -314,7 +314,7 @@ export default function VideoCard({ video, size = "md", onDismiss, variant = "gr
   }
 
   // ── Grid variant ─────────────────────────────────────────────────────────
-  const avatarUrl = channelMeta?.thumbnail_url ?? null;
+  const avatarUrl = video.channel_thumbnail_url ?? channelMeta?.thumbnail_url ?? null;
   const avatarLetter = video.channel_name?.[0]?.toUpperCase() ?? "?";
 
   return (