feat(store): NewestUnsummarizedVideoIDs for the onboarding cap

Returns up to limit of a user's newest videos (published_at DESC, NULLS LAST)
that have no summary yet. RLS-scoped via withUser — the test proves a second
user's newer video never leaks. Drives the connect-time onboarding burst
(Feature 1); the caller routes each through the shared rate gate.

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

internal/adapters/store/videos_test.go

@@ -81,3 +81,35 @@ func TestUpsertVideo_PerUserIsolation(t *testing.T) {
 
 	require.NotEqual(t, idA, idB, "same provider video for two users must be two distinct rows")
 }
+
+func TestNewestUnsummarizedVideoIDs(t *testing.T) {
+	ctx := context.Background()
+	s := newStore(t)
+	resetDB(t, rawPool(t))
+
+	mk := func(user, pid string, day int) string {
+		v := ytVideo(user, pid, pid)
+		v.PublishedAt = time.Date(2026, 6, day, 12, 0, 0, 0, time.UTC)
+		id, err := s.UpsertVideo(ctx, v)
+		require.NoError(t, err)
+		return id
+	}
+
+	_ = mk(userA, "a1vid000001", 1)
+	id2 := mk(userA, "a2vid000002", 2)
+	id3 := mk(userA, "a3vid000003", 3)
+	id4 := mk(userA, "a4vid000004", 4)
+	mk(userB, "b1vid000009", 9) // userB's newest — must never leak via RLS
+
+	// The newest (v4) is summarized, so it's excluded from "unsummarized".
+	require.NoError(t, s.Deliver(ctx, summary(userA, id4, "done")))
+
+	// Cap 2, newest-first unsummarized: v3 then v2 (v4 excluded; userB excluded).
+	got, err := s.NewestUnsummarizedVideoIDs(ctx, userA, 2)
+	require.NoError(t, err)
+	require.Equal(t, []string{id3, id2}, got)
+
+	none, err := s.NewestUnsummarizedVideoIDs(ctx, userA, 0)
+	require.NoError(t, err)
+	require.Empty(t, none, "limit 0 returns nothing")
+}