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/library/scanrun.go

@@ -100,11 +100,7 @@ func RunScan(
 		var lastStats Stats
 		libPub := newStagePublisher(ctx, 500, time.Second,
 			func() []byte {
-				blob, _ := json.Marshal(LibraryStageTallies{
-					Scanned: lastStats.Scanned, Added: lastStats.Added,
-					Updated: lastStats.Updated, Skipped: lastStats.Skipped,
-					Errored: lastStats.Errored,
-				})
+				blob, _ := json.Marshal(LibraryStageTallies(lastStats))
 				return blob
 			},
 			func(blob []byte) error {
@@ -131,10 +127,7 @@ func RunScan(
 		var lastRes BackfillMBIDsResult
 		bfPub := newStagePublisher(ctx, 500, time.Second,
 			func() []byte {
-				blob, _ := json.Marshal(MBIDBackfillStageTallies{
-					Processed: lastRes.Processed, Healed: lastRes.Healed,
-					Skipped: lastRes.Skipped, Duplicates: lastRes.Duplicates,
-				})
+				blob, _ := json.Marshal(MBIDBackfillStageTallies(lastRes))
 				return blob
 			},
 			func(blob []byte) error {