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/reset_case_test.rs

@@ -61,8 +61,9 @@ async fn reset_case_preserves_manual_oneliner_override() {
         &format!("/web/cases/{case_id}")
     );
 
-    // Transcript was wiped (the rest of the reset semantics still works).
-    assert!(!dir.join("2026-04-26T10-00-00Z.transcript.txt").exists());
+    // Recording metadata sidecar was wiped (the rest of the reset
+    // semantics still works).
+    assert!(!dir.join("2026-04-26T10-00-00Z.json").exists());
     // …but the Manual override is untouched.
     let bytes = std::fs::read(dir.join(ONELINER_FILENAME))
         .expect("oneliner.json must still exist after reset");