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/csrf_attack_test.rs

@@ -382,7 +382,12 @@ async fn case_recordings_page_renders_csrf_token_hidden_field() {
     let case_id = "11111111-1111-1111-1111-111111111111";
     let case_dir = seed_case(&cfg.data_path, "dr_a", case_id);
     std::fs::write(case_dir.join("2026-04-14T10-00-00Z.m4a"), b"x").unwrap();
-    std::fs::write(case_dir.join("2026-04-14T10-00-00Z.transcript.txt"), b"hi").unwrap();
+    common::seed_recording_meta(
+        &case_dir,
+        "2026-04-14T10-00-00Z",
+        common::Transcript::Content { text: "hi".into() },
+        None,
+    );
 
     let (app, store) = doctate_server::create_router_and_session_store(cfg);
     let (cookie, csrf) = login_with_csrf(&app, &store, "dr_a", "s").await;