refactor: Centralize all mock data into a dedicated directory

[AlAfiz]

Description

Over the course of development, we have created various dummy data arrays scattered directly inside our route controllers.
This makes the codebase messy and hard to transition when we finally connect a real PostgreSQL database.
We need to extract all hardcoded arrays (tokens, history, leaderboards) into separate files within a mock-db folder.
The controllers should simply require() these files, simulating an actual database query.

Requirements

• Create a mock-db/ directory at the root of the project.
• Create files like mock-db/tokens.js and mock-db/leaderboard.js and move the dummy arrays there.
• Export the arrays using module.exports.
• Refactor the route controllers to import this data rather than defining it inline.