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`.

src/db/seeds/02_socias.js

@@ -0,0 +1,12 @@
+exports.seed = function(knex) {
+  // Deletes ALL existing entries
+  return knex('socias').del()
+    .then(function () {
+      // Inserts seed entries
+      return knex('socias').insert([
+        {nombre: 'Ana', apellido: 'García', fecha_ingreso: '2022-01-15', id_sucursal: 1, activo: true},
+        {nombre: 'Carla', apellido: 'Lopez', fecha_ingreso: '2021-06-20', id_sucursal: 1, activo: true},
+        {nombre: 'Maria', apellido: 'Hernandez', fecha_ingreso: '2023-03-10', id_sucursal: 2, activo: false}
+      ]);
+    });
+};