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/tests/common/seed.rs

@@ -31,7 +31,7 @@ pub fn seed_case(data_path: &Path, slug: &str, case_id: &str) -> PathBuf {
 /// `transcript`:
 /// - `None` → no sidecar written (recording stays in `Pending` state).
 /// - `Some(text)` with whitespace-only `text` → sidecar with
-///   `Transcript::Silent` (matches the old `transcript.txt = ""` rule).
+///   `Transcript::Silent` (mirrors the worker's classification rule).
 /// - `Some(text)` non-empty → sidecar with `Transcript::Content`.
 ///
 /// Returns the audio filename (no path) so the caller can feed it back
@@ -79,8 +79,7 @@ pub fn seed_recording_meta(
 
 /// Map a free-text transcript to the `Transcript` variant the worker
 /// would have produced for it: whitespace-only → `Silent`, otherwise
-/// `Content`. Mirrors the old `transcript.txt = "" ⇒ Silent` rule so
-/// existing tests keep their meaning.
+/// `Content`. Matches `worker::run`'s post-Whisper classification.
 fn transcript_for(text: &str) -> Transcript {
     if text.trim().is_empty() {
         Transcript::Silent