feat(store): expose provider_video_id on SummaryRow

Read-only addition (column + field + scan) so the web layer can build a
video embed URL. No write-path or restructuring.

internal/adapters/store/reads.go

@@ -26,6 +26,7 @@ var ErrNotFound = errors.New("store: summary not found")
 // JOIN source — callers already fall back gracefully on an empty Channel.
 type SummaryRow struct {
 	VideoID         string
+	ProviderVideoID string    // videos.provider_video_id; empty when no videos row
 	Title           string    // videos.title; empty when no videos row
 	Channel         string    // videos.provider for now; empty when no videos row
 	URL             string    // videos.url; empty when no videos row
@@ -45,6 +46,7 @@ type SummaryRow struct {
 // crosses users and a missing videos row yields nulls, not a dropped summary.
 const selectSummary = `
 	SELECT s.video_id,
+	       COALESCE(v.provider_video_id, ''),
 	       COALESCE(v.title, ''),
 	       COALESCE(v.provider, ''),
 	       COALESCE(v.url, ''),
@@ -135,6 +137,7 @@ func scanSummaryRow(rows pgx.Row) (SummaryRow, error) {
 	)
 	if err := rows.Scan(
 		&row.VideoID,
+		&row.ProviderVideoID,
 		&row.Title,
 		&row.Channel,
 		&row.URL,