feat(server): scanner + DeleteTrackFile write library_changes

Wires sync.LogChange into the library mutation sites so /api/library/sync
reflects upserts and deletes.

Architectural pivot: LogChange's signature is now (ctx, dbq.DBTX, ...) so
it works with both *pgxpool.Pool and pgx.Tx. The scanner doesn't run
mutations in explicit transactions, so it pool-binds; delete.go matches.
Tx-bound callers (likes/playlists in subsequent commits) keep atomicity.

Also: sync.FormatUUID centralizes the pgtype.UUID → canonical string
conversion that both the scanner and the sync handler need; library_sync.go
now uses it instead of a local copy.

Best-effort logging on scanner failures (Warn, don't fail the scan): a
LogChange error after a successful upsert is rare and self-healing — the
next scan that touches the entity re-emits the change.

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

internal/api/library_sync_test.go

@@ -157,10 +157,10 @@ func TestLibrarySync_LikeRowScopedToUser(t *testing.T) {
 	ctx := context.Background()
 	tx, _ := pool.Begin(ctx)
 	_ = sync.LogChange(ctx, tx, sync.EntityLikeTrack,
-		sync.EncodeLikeID(uuidToString(u.ID), "11111111-1111-1111-1111-111111111111"),
+		sync.EncodeLikeID(sync.FormatUUID(u.ID), "11111111-1111-1111-1111-111111111111"),
 		sync.OpUpsert)
 	_ = sync.LogChange(ctx, tx, sync.EntityLikeTrack,
-		sync.EncodeLikeID(uuidToString(other.ID), "22222222-2222-2222-2222-222222222222"),
+		sync.EncodeLikeID(sync.FormatUUID(other.ID), "22222222-2222-2222-2222-222222222222"),
 		sync.OpUpsert)
 	_ = tx.Commit(ctx)
 
@@ -206,14 +206,3 @@ func TestSplitOnce(t *testing.T) {
 	}
 }
 
-func TestFormatUUIDBytes(t *testing.T) {
-	var b [16]byte
-	for i := range b {
-		b[i] = byte(i)
-	}
-	got := formatUUIDBytes(b)
-	want := "00010203-0405-0607-0809-0a0b0c0d0e0f"
-	if got != want {
-		t.Errorf("formatUUIDBytes = %q want %q", got, want)
-	}
-}