2565 fix announcements schema

[Giolii]

Fixes #2565

Summary

• Fixed TypeScript compilation error in announcements schema

Problem

• TypeScript compilation failed
• The required array was incomplete
• Schema required id and created_at fields which are auto-generated by the database

Solution

• Use Omit<Announcement, "id" | "created_at"> for the POST schema
• Remove nullable: true
• Remove id and created_at from properties since they're auto-generated

Test plan

• npm run typecheck passes without errors
• Creating a new announcement through the admin UI works without errors
• Test and Build passes without errors

[junjun107]

LGTM