First slice of 'follow a YouTube channel as a show'. Pure, testable resolver:
- parse_channel_url() normalizes any channel reference (@handle, /channel/UC..,
/c/.., /user/.., bare handle, with/without scheme or tab suffix) to a
canonical /videos uploads URL, and rejects non-channels (watch/playlist/
shorts/home/non-youtube).
- shape_channel() maps yt-dlp's flat-extract dict to our shape (channel meta +
recent uploads), picking best-res thumbs, parsing sparse publish dates
(timestamp or upload_date → ISO, None when flat mode omits them), filtering
null/id-less entries.
- resolve_channel() ties them together via an injectable YoutubeDL factory.
yt-dlp only (already a pinned dep), extract_flat for cheap listing. 28 seam
tests, no network. Schema bridge + API + UI are the next slices.
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