Use errgroup for bounded transcription work

docs/internal/stage-transcribe.md

@@ -16,7 +16,7 @@ Generate raw per-speaker transcripts from prepared audio using WhisperX.
 
 - discovers prepared audio from manifest inputs or canonical audio directory.
 - derives the transcript identity from the prepared `.flac` filename.
-- dispatches WhisperX requests through a bounded worker pool.
+- dispatches one WhisperX request per prepared file through a bounded errgroup.
 - validates each output as JSON.
 - writes run-local outputs then materializes canonical transcript outputs only
   after every planned request succeeds.

docs/roadmap/implementation.md

@@ -372,7 +372,7 @@ own those files.
 
 ## Stage 5 — Replace The Transcription Worker Pool With Errgroup
 
-**Status: Pending**
+**Status: Completed**
 
 ### Goal