Add PUT /announcements/:id and Harden DELETE Route

[divyamagrawal06]

Description

Implement an update endpoint for announcements and improve validation on the existing delete endpoint.

1. Create PUT /api/announcements/:id

• Allow partial updates for title, content, and priority.
• Validate input:
  • At least one field must be present.
  • priority must be one of low, medium, high.
• Use runValidators: true and return the updated document.
• (Optional: Enable timestamps: true in the Announcement.js schema).

2. Harden DELETE /api/announcements/:id

• Add ObjectId validation for the :id param.

Acceptance Criteria:

• PUT /:id updates the correct document and returns it.
• PUT /:id returns 400 if the body is empty or validation fails.
• PUT /:id returns 404 for a valid ID that doesn't exist.
• PUT and DELETE both return 400 for an invalid ObjectId format.
• Existing frontend delete functionality continues to work.