fix: make waitlist_entered_at optional in applicant schema

Fixed schema validation issue preventing 894 applicants from displaying
on the applicants page. The waitlist_entered_at field is NULL for existing
records, but the Zod schema required it to be present, causing validation
to fail silently.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: Zeff01

apps/codebility/app/home/applicants/_service/types.ts

@@ -44,7 +44,7 @@ export const newApplicantsSchema = z.object({
         quiz_completed_at: z.string().datetime({ offset: true }).nullable(),
         can_do_mobile: z.boolean().nullable(),
         commitment_signed_at: z.string().datetime({ offset: true }).nullable(),
-        waitlist_entered_at: z.string().datetime({ offset: true }).nullable(),
+        waitlist_entered_at: z.string().datetime({ offset: true }).nullable().optional(),
         signature_data: z.string().nullable(),
         created_at: z.string().datetime({ offset: true }),
         updated_at: z.string().datetime({ offset: true }),