Refactor recording metadata to JSON sidecar

Replaces the `.transcript.txt` sidecar with a structured `.json` file
for recording metadata. This change consolidates transcript text,
duration, and other potential metadata into a single, extensible JSON
object.

This also refactors the `TranscriptState` enum to better represent the
on-disk state (absence of file means pending) and the in-memory
representation. The `Transcript` enum now specifically models the
terminal outcomes of the transcriber (`Silent` or `Content`).

The commit includes updates to documentation, data structures, path
handling, and various tests to align with the new metadata format.

server/tests/silent_case_empty_test.rs

@@ -28,11 +28,11 @@ use wiremock::{Mock, MockServer, ResponseTemplate};
 
 use common::{ONELINER_FILENAME, TestConfig};
 
-/// Seed an m4a plus a silent (0-byte) transcript sidecar — the on-disk
-/// shape of a recording that whisper classified as silence.
+/// Seed an m4a plus a silent metadata sidecar — the on-disk shape of
+/// a recording that whisper classified as silence.
 fn seed_silent_recording(case_dir: &Path, stem: &str) {
     std::fs::write(case_dir.join(format!("{stem}.m4a")), b"audio").unwrap();
-    std::fs::write(case_dir.join(format!("{stem}.transcript.txt")), b"").unwrap();
+    common::seed_recording_meta(case_dir, stem, common::Transcript::Silent, None);
 }
 
 #[tokio::test]