feat(store): order video list by published_at, NULLS LAST

ListVideos now sorts summarized-first, then published_at DESC with undated
videos last, then seen_at DESC as a tiebreak (was seen_at only). Aligns the
list with the recency framing — newest content surfaces first — so the
recency-bounded feed reads coherently (UX review B2).

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

internal/adapters/store/summarize_mode_test.go

@@ -3,6 +3,7 @@ package store_test
 import (
 	"context"
 	"testing"
+	"time"
 
 	"github.com/jackc/pgx/v5/pgxpool"
 	"github.com/stretchr/testify/require"
@@ -125,6 +126,39 @@ func TestListVideosReturnsSummarizedAndUnsummarized(t *testing.T) {
 	require.True(t, byID[videoY].SummarizeRequested, "queued video carries the flag")
 }
 
+// TestListVideosOrderedByPublishedDescNullsLast: summarized videos sort first
+// (regardless of their date), then unsummarized by published_at DESC with
+// undated (NULL) videos last — the recency-aligned list order (UX review B2).
+func TestListVideosOrderedByPublishedDescNullsLast(t *testing.T) {
+	ctx := context.Background()
+	s := newStore(t)
+	p := rawPool(t)
+	resetDB(t, p)
+
+	const (
+		vSummOld = "cccccccc-cccc-cccc-cccc-cccccccccccc" // summarized, oldest date
+		vNewer   = "dddddddd-dddd-dddd-dddd-dddddddddddd" // unsummarized, newest
+		vOlder   = "eeeeeeee-eeee-eeee-eeee-eeeeeeeeeeee" // unsummarized, older
+		vUndated = "ffffffff-ffff-ffff-ffff-ffffffffffff" // unsummarized, no date
+	)
+	// Deliver first so the userA row exists (the videos FK needs it); the
+	// summarized video carries the OLDEST date yet must still sort first because
+	// it is summarized, proving summarized-first dominates the date sort.
+	require.NoError(t, s.Deliver(ctx, summary(userA, vSummOld, "body")))
+	seedVideo(t, p, userA, vSummOld, "Summarized Old", "youtube", "https://s", time.Date(2025, 1, 1, 0, 0, 0, 0, time.UTC))
+	seedVideo(t, p, userA, vNewer, "Newer", "youtube", "https://n", time.Date(2026, 3, 1, 0, 0, 0, 0, time.UTC))
+	seedVideo(t, p, userA, vOlder, "Older", "youtube", "https://o", time.Date(2026, 1, 1, 0, 0, 0, 0, time.UTC))
+	seedVideo(t, p, userA, vUndated, "Undated", "youtube", "https://u", time.Time{})
+
+	rows, err := s.ListVideos(ctx, userA, 50)
+	require.NoError(t, err)
+	require.Len(t, rows, 4)
+
+	got := []string{rows[0].VideoID, rows[1].VideoID, rows[2].VideoID, rows[3].VideoID}
+	require.Equal(t, []string{vSummOld, vNewer, vOlder, vUndated}, got,
+		"summarized first, then published_at DESC, NULL dates last")
+}
+
 func TestListVideosIsUserScoped(t *testing.T) {
 	ctx := context.Background()
 	s := newStore(t)