refactor: use GraphQL string scalar for KSUID transit

Marshal and unmarshal both use the base62 KSUID string, replacing the
legacy JSON object wrapper and aligning gqlgen scalar behavior.

Co-authored-by: Cursor <cursoragent@cursor.com>

ksuid_binary.go

@@ -9,8 +9,8 @@ import (
 )
 
 // BinaryKSUID wraps segmentio/ksuid for binary-column storage (BINARY(20), BYTEA).
-// SQL Scan/Value use the raw 20-byte encoding. GraphQL transit uses the shared
-// string JSON transport (see ksuid_gql.go).
+// SQL Scan/Value use the raw 20-byte encoding. GraphQL transit uses a string
+// scalar (base62 KSUID).
 type BinaryKSUID struct {
 	ksuid.KSUID
 }