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/silent_case_empty_test.rs

@@ -2,11 +2,11 @@
 //! (whisper classified as silence) must produce an `OnelinerState::Empty`
 //! automatically, so the UI doesn't stick on "Generating" forever.
 //!
-//! The historical bug path: whisper wrote a 0-byte `.transcript.txt`,
-//! `update_oneliner` saw no content and returned early without persisting
-//! any state, and the UI's `compute_oneliner_display` collapsed the
-//! Silent file onto "transcribed → Generating" (missing state). The fix
-//! makes `update_oneliner` settle the case to `OnelinerState::Empty`
+//! The historical bug path: whisper wrote a sidecar marking the recording
+//! as silent, `update_oneliner` saw no content and returned early without
+//! persisting any state, and the UI's `compute_oneliner_display` collapsed
+//! the Silent state onto "transcribed → Generating" (missing state). The
+//! fix makes `update_oneliner` settle the case to `OnelinerState::Empty`
 //! when no content transcript exists and no recording is still Pending.
 
 mod common;