Video tiles: a frame even when the size check fails; retry it later

Efrat's clip (blossom.primal.net → r2a.primal.net) showed a blank tile on
a phone: the 64 KB probe failed there (slow connection), the failure was
kept for good, and a tile with a failed probe never tried for a frame.

- A failed probe is dated and asked again after an hour (it was kept
  forever; old 'failed: true' entries are retried). The probe waits 10 s.
- A tile whose probe failed still tries for a first frame, giving up and
  letting go of the file after 8 s.
- Primal's CDN lets scripts read the bytes but not Content-Range, so the
  size was unknown: a HEAD's Content-Length (readable by any page) fills
  it in.
- No size in imeta but a bitrate and duration (Primal posts those): the
  size is estimated from them. 'dim 1080.0x1920.0' is understood.
- A tile with nothing to show is dark, like a video, not a pale gap.

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01ULQZS9MQa6iAsXnrJUD9Du

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