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.up.sql

@@ -0,0 +1,17 @@
+-- Migration 007: per-video transcript fetch status, for rate-limit backoff.
+--
+-- transcript_status records the outcome of the last transcript attempt:
+--   NULL          = not yet attempted
+--   'none'        = checked, no usable transcript (permanent — SourceNone)
+--   'fetched'     = transcript resolved and summarized (summary_id not null)
+--   'rate_limited'= the caption endpoint returned 429; retry after a backoff window
+--
+-- rate_limited_at stamps WHEN the 429 was seen, so the runner can skip re-fetching
+-- a still-throttled video until NOW() - rate_limited_at exceeds TAPIR_FETCH_BACKOFF.
+-- It is cleared (set NULL) whenever the status moves off 'rate_limited'.
+--
+-- No RLS policy changes needed: videos already has ENABLE + FORCE ROW LEVEL
+-- SECURITY (migration 003) with the videos_isolation policy. New columns inherit
+-- that protection automatically.
+ALTER TABLE videos ADD COLUMN transcript_status TEXT;
+ALTER TABLE videos ADD COLUMN rate_limited_at TIMESTAMPTZ;