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/delete_recording_test.rs

@@ -1,7 +1,7 @@
 //! Per-recording delete endpoint: POST /web/cases/{case_id}/recordings/delete.
 //!
 //! Verifies the endpoint:
-//! - removes the audio file and its transcript / duration sidecars
+//! - removes the audio file and its `<stem>.json` metadata sidecar
 //! - invalidates derived artefacts (oneliner.json, document.md, analysis_input.json)
 //! - leaves other recordings in the same case untouched
 //! - rejects path-traversal filenames with 400
@@ -68,26 +68,18 @@ async fn delete_recording_removes_file_and_sidecars_and_invalidates_derived() {
         "delete should redirect back to the recording list, not the case list"
     );
 
-    // Victim + its sidecars are gone.
+    // Victim + its sidecar are gone.
     assert!(!case_dir.join(&victim).exists(), "m4a should be deleted");
     assert!(
-        !case_dir
-            .join("2026-04-19T10-00-00Z.transcript.txt")
-            .exists(),
-        "transcript sidecar should be deleted"
-    );
-    assert!(
-        !case_dir.join("2026-04-19T10-00-00Z.duration.txt").exists(),
-        "duration sidecar should be deleted"
+        !case_dir.join("2026-04-19T10-00-00Z.json").exists(),
+        "metadata sidecar should be deleted"
     );
 
     // Survivor is intact.
     assert!(case_dir.join(&survivor).exists(), "other m4a must remain");
     assert!(
-        case_dir
-            .join("2026-04-19T11-00-00Z.transcript.txt")
-            .exists(),
-        "other transcript must remain"
+        case_dir.join("2026-04-19T11-00-00Z.json").exists(),
+        "other metadata sidecar must remain"
     );
 
     // Derived artefacts are invalidated.