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, 7 insertions, 2 deletions

diff --git a/tests/test-transcription-status-and-commands.el b/tests/test-transcription-status-and-commands.el
index 90841e70..7c796de0 100644
--- a/tests/test-transcription-status-and-commands.el
+++ b/tests/test-transcription-status-and-commands.el
@@ -179,13 +179,18 @@
     (should-error (cj/transcribe-audio-at-point) :type 'user-error)))
 
 (ert-deftest test-tx-transcribe-audio-at-point-normal-delegates ()
-  "Normal: with a file at point, delegates to `cj/transcribe-audio'."
+  "Normal: with a file at point, delegates to `cj/transcribe-media'.
+
+`cj/transcribe-audio-at-point' is now a `defalias' for
+`cj/transcribe-media-at-point', which hands off to
+`cj/transcribe-media' (no longer the old audio-only command).  The
+stub still pins behavior by name."
   (let ((handed-off nil))
     (cl-letf (((symbol-function 'derived-mode-p)
                (lambda (&rest modes) (memq 'dired-mode modes)))
               ((symbol-function 'dired-get-filename)
                (lambda (&rest _) "/tmp/recording.wav"))
-              ((symbol-function 'cj/transcribe-audio)
+              ((symbol-function 'cj/transcribe-media)
                (lambda (f) (setq handed-off f))))
       (cj/transcribe-audio-at-point))
     (should (equal handed-off "/tmp/recording.wav"))))