feat(transcription): extend dired T to transcribe videos via ffmpeg, with tests

Pressing `T' in dired/dirvish on an audio file already transcribed
it; on a video file it bounced with "Not an audio file".  Real
recordings ship as .mp4 / .mkv at least as often as raw .m4a, so
the one-key flow ended at the wrong place.

Pipeline now:
- audio path -> direct into `cj/--start-transcription-process'
  (unchanged).
- video path -> async ffmpeg extracts the audio track to a temp
  .mp3 under `temporary-file-directory' (libmp3lame, VBR q:a 4,
  ~165kbps -- right size for speech, accepted by every backend),
  then transcribes that file with the temp marked for cleanup
  after the transcription sentinel fires.

Surface changes:
- `cj/video-file-extensions' added to user-constants.el (mp4, mkv,
  mov, webm, avi, m4v, wmv, flv, mpg, mpeg, 3gp, ogv).
- New predicates `cj/--video-file-p' / `cj/--media-file-p'.
- New `cj/--extract-audio-from-video' (async ffmpeg with success
  callback; surfaces `cj/--notify' on failure; user-errors if
  ffmpeg isn't on PATH).
- `cj/--start-transcription-process' gains optional `cleanup-file'.
  Sentinel deletes it after the existing logic runs.  Backwards
  compatible -- the audio flow doesn't pass it.
- `cj/transcribe-audio' renamed to `cj/transcribe-media' (dispatcher
  on audio vs video).  `cj/transcribe-audio-at-point' renamed to
  `cj/transcribe-media-at-point'.  Both old names kept as
  `defalias' so M-x history and any external references still work.
- `T' in dired-mode-map + dirvish-mode-map points at
  `cj/transcribe-media-at-point'.
- Module commentary USAGE block updated.

15 new ERT tests in `tests/test-transcription-video.el' cover the
predicates (happy/boundary/error), ffmpeg invocation (correct args
+ missing-ffmpeg path), the dispatcher (audio direct, video via
extraction, non-media rejected), the aliases, and the T binding.
One existing test in `test-transcription-status-and-commands.el'
updated to stub the new delegate name.

Verified locally that ffmpeg is on PATH with libmp3lame, and that
the exact arg list my code uses produces a valid MP3 from a
synthetic test video.

1 files changed, 6 insertions, 0 deletions

diff --git a/modules/user-constants.el b/modules/user-constants.el
index b1f6016b..21e141a5 100644
--- a/modules/user-constants.el
+++ b/modules/user-constants.el
@@ -56,6 +56,12 @@ Used by cursor color, modeline, and other UI elements.")
   "File extensions recognized as audio files.
 Used by transcription module and other audio-related functionality.")
 
+(defvar cj/video-file-extensions
+  '("mp4" "mkv" "mov" "webm" "avi" "m4v" "wmv" "flv" "mpg" "mpeg" "3gp" "ogv")
+  "File extensions recognized as video files.
+Used by transcription to dispatch the audio-extract step in front of
+the regular transcription pipeline.")
+
 ;; ------------------------ Directory And File Constants -----------------------
 
 ;; DIRECTORIES