dc12e08a

By: Michael Lynch <git@mtlynch.io>

Make video transcoding durable and tune the encoder

Rework background video transcoding so it survives restarts and fails
visibly, and tune ffmpeg for the resolutions we actually serve.

- Treat media_items 'processing' rows as a durable queue: the worker
  drains the DB (ReadNextProcessingVideo) and resumes any video left
  processing by a crash or restart. The HTTP handler no longer blocks
  on a full in-memory channel.
- Retain original video uploads under --originals-dir (defaults beside
  the DB) as both the worker's transcode source and the record DESIGN
  requires, instead of a deleted temp file.
- Add a 'failed' media status so a deterministic transcode error shows
  the uploader a re-upload notice rather than a perpetual "processing"
  one. Crash -> still processing (retried); error -> failed.
- Shut down gracefully: cancel the in-flight ffmpeg on SIGTERM via
  exec.CommandContext and an http.Server/Server.Shutdown handshake; the
  cancelled item stays processing and resumes next start.
- Cap served video to a 2560px box (no upscale), add VP9 speed flags,
  and seek past the blank opening frame for thumbnails. Missing ffmpeg
  or ffprobe surfaces at conversion time, not startup.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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