refactor: Convert spotlight.js to TypeScript and remove object mutations

Background

The module apps/meteor/server/lib/spotlight.js powers the user and room autocomplete search functionality.

It appears to be one of the remaining plain JavaScript files in server/lib/, which is otherwise TypeScript-based.

Observations

• The file is ~280 lines of untyped JavaScript.
• processLimitAndUsernames() mutates options.limit and the usernames array in-place.
• fetchRooms() uses delete room.lastMessage, which mutates returned MongoDB documents.
• There are currently no unit tests covering this module.

Proposal

I would like to prepare a refactor PR that:

1. Renames spotlight.js → spotlight.ts
2. Introduces explicit TypeScript interfaces for search parameters and return types
3. Replaces in-place mutations with pure transformations
4. Avoids delete on return objects by using object destructuring
5. Preserves all existing behavior (no functional changes)

Start by reading apps/meteor/server/lib/spotlight.js, focusing on processLimitAndUsernames() and fetchRooms(). Confirm the current user and room autocomplete behavior before renaming it to spotlight.ts. Done means explicit TypeScript interfaces, pure transformations instead of object or array mutations, no delete on returned documents, and preserved behavior.