fix: settle silent-only cases to Empty oneliner state

Introduce `TranscriptState { Pending, Silent, Content }` in
doctate-common as the canonical three-way state of a recording's
`.transcript.txt` sidecar. Previously each call-site projected the
raw `Option<String>` / `.exists()` onto its own 2-state view and the
projections disagreed: the UI treated a 0-byte silent transcript like
`Content` while the oneliner worker treated it like `Pending`,
leaving silent-only cases stuck on "generiere Titel …" forever with
no persisted oneliner.json.

`update_oneliner` now settles a silent-only case to
`OnelinerState::Empty` when no `Content` transcript exists and no
recording is still `Pending`, so the UI resolves to "unbenannt" and
recovery treats it as terminal.

Single reader: `paths::read_transcript_state`. All call-sites
(scan_recordings, compute_oneliner_display, has_pending_recordings,
all_transcripts_joined, read_recordings, enqueue_pending_for_user,
scan_m4as, cases_needing_oneliner_in, case_recordings.html) go
through the same typed abstraction and must handle `Silent` via
exhaustive match.

Regression tests:
- silent_case_empty_test: heal path settles silent-only case to
  Empty without calling Ollama
- case_page_silent_only_shows_empty_not_generating: UI renders
  "unbenannt", not "generiere Titel …"

server/templates/case_recordings.html

@@ -117,13 +117,11 @@ try {
 <div class="failed">Transkription fehlgeschlagen — .failed-Suffix entfernen zum Erneut-Versuchen.</div>
 {% else %}
 {% match rec.transcript %}
-{% when Some with (t) %}
-{% if t.is_empty() %}
-<div class="pending">(stille Aufnahme)</div>
-{% else %}
+{% when TranscriptState::Content with (t) %}
 <div class="transcript">{{ t }}</div>
-{% endif %}
-{% when None %}
+{% when TranscriptState::Silent %}
+<div class="pending">(stille Aufnahme)</div>
+{% when TranscriptState::Pending %}
 {% if transcribe_busy %}
 <div class="pending">Transkription läuft…</div>
 {% else %}