feat(discovery): persist video duration_s instead of discarding it (ADR-028)

ADR-023's filterLowValue already fetches each candidate's duration via the
cheap videos.list quota call to drop Shorts/live, then threw it away — the
videos.duration_s column (migration 001) was never written. Carry it onto the
kept domain.Video and have UpsertVideo persist it, COALESCE-preserving a known
value so an unknown (0) re-upsert never clobbers it (the channel_title backfill
stance, migration 014). This is the enabling change for length-aware burst
selection. No new migration — the column already exists.

internal/adapters/store/videos_test.go

@@ -52,6 +52,36 @@ func TestUpsertVideo_ReturnsStableID(t *testing.T) {
 	require.Equal(t, 1, count, "must not duplicate the row")
 }
 
+func TestUpsertVideo_PersistsAndPreservesDuration(t *testing.T) {
+	ctx := context.Background()
+	s := newStore(t)
+	resetDB(t, rawPool(t))
+
+	// First upsert carries a known duration (ADR-028: discovery enriches it).
+	v := ytVideo(userA, "dur0000001x", "with duration")
+	v.DurationSeconds = 750
+	id, err := s.UpsertVideo(ctx, v)
+	require.NoError(t, err)
+
+	p := rawPool(t)
+	readDuration := func() *int {
+		var d *int
+		require.NoError(t, p.QueryRow(ctx, `SELECT duration_s FROM videos WHERE id = $1`, id).Scan(&d))
+		return d
+	}
+	require.NotNil(t, readDuration())
+	require.Equal(t, 750, *readDuration(), "duration must persist")
+
+	// A later upsert that does NOT know the duration (0) must not clobber it —
+	// the channel_title backfill stance (migration 014): COALESCE-preserve.
+	v2 := ytVideo(userA, "dur0000001x", "title updated, duration unknown")
+	v2.DurationSeconds = 0
+	_, err = s.UpsertVideo(ctx, v2)
+	require.NoError(t, err)
+	require.NotNil(t, readDuration(), "a 0/unknown re-upsert must not erase a known duration")
+	require.Equal(t, 750, *readDuration())
+}
+
 func TestUpsertVideo_IDMatchesSummaryDedup(t *testing.T) {
 	ctx := context.Background()
 	s := newStore(t)