This commit establishes the database foundation for the HR Platform.
Key changes include:
- Defined the complete data schema in `docs/API_CONTRACTS.md` to serve as a single source of truth.
- Integrated `knex.js` with `sqlite3` to manage the database connection and schema.
- Implemented a version-controlled migration system and created initial migrations for all required tables.
- Created seed files to populate the database with sample data for development.
- Addressed security feedback by using `bcrypt` to hash user passwords in the seed data and adding the SQLite database file to `.gitignore`.
