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`.

package.json

@@ -1,8 +1,12 @@
 {
   "dependencies": {
-    "express": "^4.19.2"
+    "bcrypt": "^6.0.0",
+    "express": "^4.19.2",
+    "knex": "^3.1.0",
+    "sqlite3": "^5.1.7"
   },
   "scripts": {
-    "start": "node src/index.js"
+    "start": "node src/index.js",
+    "knex": "knex"
   }
 }