fix(auth): convert wsdcId to string for Prisma compatibility

backend/src/controllers/auth.js

@@ -57,7 +57,7 @@ async function register(req, res, next) {
         OR: [
           { email },
           { username },
-          ...(wsdcId ? [{ wsdcId }] : []),
+          ...(wsdcId ? [{ wsdcId: String(wsdcId) }] : []),
         ],
       },
     });
@@ -95,7 +95,7 @@ async function register(req, res, next) {
         passwordHash,
         firstName,
         lastName,
-        wsdcId,
+        wsdcId: wsdcId ? String(wsdcId) : null,
         verificationToken,
         verificationCode,
         verificationTokenExpiry,