Centralize status string constants across API routes #8

Problem

Status strings (`'pending'`, `'approved'`, `'published'`, `'rejected'`, `'cancelled'`) are scattered as bare string literals across 10+ API routes and admin pages:

• `events/submit.ts`: `status: 'pending'`
• `events-staged/approve.ts`: `status: 'approved'`
• `events-staged/reject.ts`: `status: 'rejected'`
• `announcements/submit.ts`: `status: 'pending'`
• `announcements/approve.ts`: `status: 'published'`
• Admin dashboard JS: status badge rendering with string comparisons

Proposed Solution

Add status enums to `src/lib/constants.ts`:
```typescript
export const EVENT_STATUS = {
PENDING: 'pending',
APPROVED: 'approved',
REJECTED: 'rejected',
CANCELLED: 'cancelled',
} as const;

export const ANNOUNCEMENT_STATUS = {
PENDING: 'pending',
PUBLISHED: 'published',
REJECTED: 'rejected',
} as const;
```

Impact

Prevents typo bugs, enables autocomplete, single source of truth.