fix(go): collapse struct-literal copies to type conversions (S1016)

staticcheck S1016 in the new golangci-lint v2 flagged four sites
copying field-by-field between two types with identical struct
shapes. Direct type conversion is the canonical form:

- internal/library/scanrun.go:
  - LibraryStageTallies copy from Stats → LibraryStageTallies(lastStats)
  - MBIDBackfillStageTallies copy from BackfillMBIDsResult →
    MBIDBackfillStageTallies(lastRes)
- internal/recommendation/home.go:
  - dbq.ListRediscoverAlbumsForUserRow copy from the Fallback row
    type → dbq.ListRediscoverAlbumsForUserRow(r)
  - Same pattern for the Artists rediscover pair.

No behavior change; the underlying struct shapes are identical
(staticcheck verified the conversion is valid). Net -18 +4 lines.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/recommendation/home.go

@@ -145,10 +145,7 @@ func loadRediscoverAlbums(ctx context.Context, q *dbq.Queries, userID pgtype.UUI
 		if _, dup := seen[r.Album.ID]; dup {
 			continue
 		}
-		primary = append(primary, dbq.ListRediscoverAlbumsForUserRow{
-			Album:      r.Album,
-			ArtistName: r.ArtistName,
-		})
+		primary = append(primary, dbq.ListRediscoverAlbumsForUserRow(r))
 		if len(primary) >= HomeRediscoverLimit {
 			break
 		}
@@ -181,11 +178,7 @@ func loadRediscoverArtists(ctx context.Context, q *dbq.Queries, userID pgtype.UU
 		if _, dup := seen[r.Artist.ID]; dup {
 			continue
 		}
-		primary = append(primary, dbq.ListRediscoverArtistsForUserRow{
-			Artist:       r.Artist,
-			CoverAlbumID: r.CoverAlbumID,
-			AlbumCount:   r.AlbumCount,
-		})
+		primary = append(primary, dbq.ListRediscoverArtistsForUserRow(r))
 		if len(primary) >= HomeRediscoverLimit {
 			break
 		}