Refactor transcript file handling to use JSON metadata

This commit changes the way transcriptions are stored and accessed.
Instead of using plain text files (`.transcript.txt`), transcriptions
will now be part of a JSON metadata file (`<stem>.json`). This allows
for richer metadata to be stored alongside the transcript, such as
duration, and provides a more robust mechanism for tracking
transcription states.

The changes include:
- Updating documentation and code to reflect the new `.json` file
  extension.
- Modifying file handling logic to read and write JSON metadata.
- Adjusting tests to accommodate the new file format.

server/src/paths.rs

@@ -176,10 +176,9 @@ pub async fn delete_oneliner_unless_manual(
 /// to it with the `.json` extension.
 ///
 /// This is the single reader of that sidecar's transcript field.
-/// Every call-site that used to do
-/// `tokio::fs::read_to_string(...).ok()` or
-/// `with_extension("transcript.txt").exists()` should go through here
-/// so the three-state semantics stay consistent across the codebase.
+/// Every "is this recording transcribed?" check should go through
+/// here so the three-state semantics (Pending / Silent / Content)
+/// stay consistent across the codebase.
 pub async fn read_transcript_state(audio_stem_path: &Path) -> TranscriptState {
     TranscriptState::from_meta(read_recording_meta(audio_stem_path).await.as_ref())
 }