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_override_test.rs

@@ -250,11 +250,14 @@ async fn early_latch_skips_llm_call_when_manual_set() {
     // Also seed a transcript so update_oneliner has content to pass to LLM
     // (would call without latch).
     std::fs::write(case_dir.join("2026-04-26T11-00-00Z.m4a"), b"audio-bytes").unwrap();
-    std::fs::write(
-        case_dir.join("2026-04-26T11-00-00Z.transcript.txt"),
-        "Patient hat Fieber.",
-    )
-    .unwrap();
+    common::seed_recording_meta(
+        &case_dir,
+        "2026-04-26T11-00-00Z",
+        common::Transcript::Content {
+            text: "Patient hat Fieber.".into(),
+        },
+        None,
+    );
 
     let locks = OnelinerLocks::new();
     let events_tx = doctate_server::events::channel();
@@ -316,11 +319,14 @@ async fn re_check_drops_llm_result_when_manual_appears_during_regen() {
 
     // Seed transcript so update_oneliner enters the LLM branch.
     std::fs::write(case_dir.join("2026-04-26T11-00-00Z.m4a"), b"audio-bytes").unwrap();
-    std::fs::write(
-        case_dir.join("2026-04-26T11-00-00Z.transcript.txt"),
-        "Patient klagt über Schwindel.",
-    )
-    .unwrap();
+    common::seed_recording_meta(
+        &case_dir,
+        "2026-04-26T11-00-00Z",
+        common::Transcript::Content {
+            text: "Patient klagt über Schwindel.".into(),
+        },
+        None,
+    );
 
     let locks = OnelinerLocks::new();
     let events_tx = doctate_server::events::channel();