feat(store): migration 007 — per-video transcript status

Adds videos.transcript_status (NULL|none|fetched|rate_limited) and
videos.rate_limited_at, so the runner can record a 429 and skip re-fetching a
still-throttled video until a backoff window elapses. Columns inherit the
existing videos RLS policy (migration 003); no policy change needed.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/adapters/store/migrations/007_transcript_status.down.sql

@@ -0,0 +1,2 @@
+ALTER TABLE videos DROP COLUMN IF EXISTS rate_limited_at;
+ALTER TABLE videos DROP COLUMN IF EXISTS transcript_status;