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_string.go

@@ -9,8 +9,8 @@ import (
 )
 
 // StringKSUID wraps segmentio/ksuid for text-column storage (VARCHAR, TEXT).
-// SQL Scan/Value use the base62 string encoding. GraphQL transit uses the
-// shared string JSON transport (see ksuid_gql.go).
+// SQL Scan/Value use the base62 string encoding. GraphQL transit uses a string
+// scalar (base62 KSUID).
 type StringKSUID struct {
 	ksuid.KSUID
 }