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/templates/case_recordings.html

@@ -164,8 +164,8 @@ try {
 <div class="failed">Transkription fehlgeschlagen</div>
 {% else %}
 {% match rec.transcript %}
-{% when TranscriptState::Content with (t) %}
-<div class="transcript">{{ t }}</div>
+{% when TranscriptState::Content with { text } %}
+<div class="transcript">{{ text }}</div>
 {% when TranscriptState::Silent %}
 <div class="pending">(stille Aufnahme)</div>
 {% when TranscriptState::Pending %}