feat(store): OnboardBurstVideoIDs — junk-avoiding burst selection (ADR-028)

Newest-first but quality-aware: excludes a video when its duration is KNOWN and
outside [minSeconds, maxSeconds], dropping Shorts and multi-hour livestream VODs
that waste a scarce caption fetch on a poor first impression. NULL/unknown
duration is kept (degrade-open) but ranked after known-good rows. 0/0 bounds
disable the filter (pure newest-first, the reversibility lever). RLS-scoped.
NewestUnsummarizedVideoIDs is left in place for callers that want pure-newest.

internal/adapters/store/videos_test.go

@@ -144,6 +144,55 @@ func TestNewestUnsummarizedVideoIDs(t *testing.T) {
 	require.Empty(t, none, "limit 0 returns nothing")
 }
 
+func TestOnboardBurstVideoIDs(t *testing.T) {
+	ctx := context.Background()
+	s := newStore(t)
+	resetDB(t, rawPool(t))
+
+	mk := func(user, pid string, day, dur int) string {
+		v := ytVideo(user, pid, pid)
+		v.PublishedAt = time.Date(2026, 6, day, 12, 0, 0, 0, time.UTC)
+		v.DurationSeconds = dur // 0 == unknown (NULL)
+		id, err := s.UpsertVideo(ctx, v)
+		require.NoError(t, err)
+		return id
+	}
+
+	good1 := mk(userA, "good0000001", 5, 600)    // 10m, newest known-good
+	tooLong := mk(userA, "toolong0001", 4, 20000) // > maxSeconds -> dropped
+	_ = tooLong
+	tooShort := mk(userA, "tooshort001", 3, 30) // < minSeconds -> dropped
+	_ = tooShort
+	unknown := mk(userA, "unknown0001", 2, 0)  // NULL duration -> kept, ranked last
+	good2 := mk(userA, "good0000002", 1, 800)  // known-good but oldest
+	mk(userB, "bvid0000009", 9, 600)           // userB -> must not leak via RLS
+
+	const minSec, maxSec = 60, 14400
+
+	// Known-good ranked before unknown, each newest-first within its group; the
+	// too-long and too-short videos are excluded by their known duration.
+	got, err := s.OnboardBurstVideoIDs(ctx, userA, 5, minSec, maxSec)
+	require.NoError(t, err)
+	require.Equal(t, []string{good1, good2, unknown}, got,
+		"known-good first (newest-first), then unknown-duration; junk excluded")
+
+	// Cap is honoured.
+	capped, err := s.OnboardBurstVideoIDs(ctx, userA, 2, minSec, maxSec)
+	require.NoError(t, err)
+	require.Equal(t, []string{good1, good2}, capped)
+
+	// Bounds disabled (0/0) == pure newest-first, nothing excluded.
+	all, err := s.OnboardBurstVideoIDs(ctx, userA, 10, 0, 0)
+	require.NoError(t, err)
+	require.ElementsMatch(t, []string{good1, tooLong, tooShort, unknown, good2}, all,
+		"0/0 bounds disable the duration filter (prior newest-first behaviour)")
+
+	// limit <= 0 returns nothing.
+	none, err := s.OnboardBurstVideoIDs(ctx, userA, 0, minSec, maxSec)
+	require.NoError(t, err)
+	require.Empty(t, none)
+}
+
 func TestUpsertVideoPersistsChannelAndDistinctChannels(t *testing.T) {
 	ctx := context.Background()
 	s := newStore(t)