feat: add local voice transcription pipeline

migrations/0003_async_transcription_jobs.sql

@@ -0,0 +1,8 @@
+ALTER TABLE audio_segments
+  ADD COLUMN transcription_attempts INTEGER NOT NULL DEFAULT 0
+    CHECK (transcription_attempts >= 0),
+  ADD COLUMN processing_started_at TIMESTAMPTZ;
+
+CREATE INDEX audio_segments_processing_queue_idx
+  ON audio_segments (created_at, id)
+  WHERE status = 'processing';