feat(dross): voice Phase 2b — clip retention (2.13.0)

'Keep voice clips' setting (default off). When on, /api/voice/transcribe
saves the audio (0600) to the owner-only ZFS subvol at /var/lib/void/
voice-clips (CT 311 mp0, replicated to Z3) + a voice_clips row (migration
029, transcript+metadata in void-db). New clips list/play/delete API +
Settings UI. Storage path is configurable (VOICE_CLIPS_DIR).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lib/db/migrations/029_voice_clips.sql

@@ -0,0 +1,14 @@
+-- 029_voice_clips.sql
+-- Optional retained Dross voice clips (when the "Keep voice clips" setting is on).
+-- Transcript + metadata here (durable, HA-replicated); audio bytes live as files
+-- on the owner-only ZFS subvol mounted at /var/lib/void/voice-clips.
+CREATE TABLE voice_clips (
+  id          uuid PRIMARY KEY DEFAULT gen_random_uuid(),
+  transcript  text NOT NULL DEFAULT '',
+  duration_ms integer,
+  bytes       bigint,
+  mime        text,
+  path        text NOT NULL,
+  created_at  timestamptz NOT NULL DEFAULT now()
+);
+CREATE INDEX idx_voice_clips_created ON voice_clips (created_at DESC);