- Extract _is_blocked(db, user, channel_id): the BlockedChannel EXISTS query was
copy-pasted in channel_detail, explore_channel, and block_channel.
- Extract _channel_summary(ch): the shared id/title/handle/thumbnail/subscriber/
video_count projection was hand-rolled in list_channels and discover_channels
(the detail dict interleaves these with extras, so it's left as-is).
Behavior-neutral. ruff clean, localdev boots healthy.
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