fix: report what the platform actually answered instead of a missing video

[juliantellez]

What. The deployed page reported a working video as deleted. This makes the boundary say what the platform actually answered, then gives each cause its own page, its own status code and its own line in the log.

Why. The reader threw the same error whenever the title came back null, so a refusal, a rate limit and a genuinely missing video all reached the reader as one 404 saying "No video with that id". Nothing was logged at that boundary, so the log said nothing either.

The cause, from the deployed function's own log. cause: bot_check, status 200, 1,412,860 bytes, has_caption_tracks: false, playability: LOGIN_REQUIRED, reason "Sign in to confirm you’re not a bot". The platform refuses the address the function calls from, and it refuses at the watch page, before the proof of origin work is reached. The same video read from a laptop returns 560 segments.

Only a missing video answers 404 now. A refusal answers 502, says the video is fine, and offers the video on the platform. The deploy check stays red while the product cannot read a transcript, and it now names the cause instead of printing a bare 404.