feat: Set up database and data models

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`.

knexfile.js

@@ -0,0 +1,16 @@
+// knexfile.js
+module.exports = {
+  development: {
+    client: 'sqlite3',
+    connection: {
+      filename: './dev.sqlite3'
+    },
+    useNullAsDefault: true,
+    migrations: {
+      directory: './src/db/migrations'
+    },
+    seeds: {
+      directory: './src/db/seeds'
+    }
+  }
+};