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/oneliner_heal_decoupled_test.rs

@@ -26,17 +26,20 @@ use wiremock::{Mock, MockServer, ResponseTemplate};
 use common::{ONELINER_FILENAME, TestConfig};
 
 fn write_transcript(case_dir: &Path) {
-    // Seed an m4a + transcript pair, matching the worker's on-disk
-    // layout. The oneliner worker iterates `.m4a` files and resolves
-    // the sidecar from the stem; a stray transcript without an m4a
-    // would be invisible to it.
+    // Seed an m4a + metadata sidecar pair, matching the worker's
+    // on-disk layout. The oneliner worker iterates `.m4a` files and
+    // resolves the sidecar from the stem; a stray sidecar without an
+    // m4a would be invisible to it.
     let stem = "2026-04-16T10-00-00Z";
     std::fs::write(case_dir.join(format!("{stem}.m4a")), b"audio").unwrap();
-    std::fs::write(
-        case_dir.join(format!("{stem}.transcript.txt")),
-        "Brustschmerz links, seit heute morgen.",
-    )
-    .unwrap();
+    common::seed_recording_meta(
+        case_dir,
+        stem,
+        common::Transcript::Content {
+            text: "Brustschmerz links, seit heute morgen.".to_owned(),
+        },
+        None,
+    );
 }
 
 #[tokio::test]