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`.

.gitignore

@@ -1,74 +1,2 @@
-# Dependencies
-/node_modules
-
-# Logs
-logs
-*.log
-npm-debug.log*
-yarn-debug.log*
-yarn-error.log*
-
-# Runtime data
-pids
-*.pid
-*.seed
-*.pid.lock
-
-# Directory for instrumented libs generated by jscoverage/JSCover
-lib-cov
-
-# Coverage directory used by tools like istanbul
-coverage
-
-# nyc test coverage
-.nyc_output
-
-# Grunt intermediate storage (http://gruntjs.com/creating-plugins#storing-temporary-files)
-.grunt
-
-# Bower dependency directory (https://bower.io/)
-bower_components
-
-# node-waf configuration
-.lock-wscript
-
-# Compiled binary addons (http://nodejs.org/api/addons.html)
-build/Release
-
-# Dependency directories
-jspm_packages/
-
-# TypeScript v1 declaration files
-typings/
-
-# Optional npm cache directory
-.npm
-
-# Optional eslint cache
-.eslintcache
-
-# Microbundle cache
-.rpt2_cache/
-.rts2_cache_cjs/
-.rts2_cache_es/
-.rts2_cache_umd/
-
-# Optional REPL history
-.node_repl_history
-
-# Output of 'npm pack'
-*.tgz
-
-# Yarn Integrity file
-.yarn-integrity
-
-# dotenv environment variables file
-.env
-.env.test
-.env.production
-
-# Mac files
-.DS_Store
-
-# VSCode files
-.vscode/
+node_modules
+*.sqlite3