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/youtube/youtube_test.go

@@ -224,6 +224,11 @@ func TestNewVideosFiltersShortsAndLive(t *testing.T) {
 	if len(vids) != 1 || vids[0].ProviderVideoID != "long1" {
 		t.Fatalf("expected only long1 to survive the filter, got %+v", vids)
 	}
+	// The duration fetched for the filter is carried onto the kept video so the
+	// store can persist it (ADR-028) instead of discarding it.
+	if vids[0].DurationSeconds != 750 {
+		t.Fatalf("kept video DurationSeconds = %d, want 750 (PT12M30S)", vids[0].DurationSeconds)
+	}
 }
 
 // TestNewVideosNoFilterWhenDisabled: MinVideoSeconds=0 keeps the pre-ADR-023