feat: Introduce knex for database migrations

- Adds knex.js to manage database schema changes systematically.
- Creates an initial migration file based on `hyungi_schema_v2.sql` to represent the current database state.
- Adds npm scripts (`db:migrate`, `db:rollback`, etc.) for easy execution of migration tasks.
- Archives legacy SQL files and old migration scripts into the `db_archive/` directory to prevent confusion and clean up the project structure.

api.hyungi.net/knexfile.js

@@ -0,0 +1,57 @@
+require('dotenv').config();
+
+module.exports = {
+  development: {
+    client: 'mysql2',
+    connection: {
+      host: process.env.DB_HOST,
+      port: process.env.DB_PORT,
+      user: process.env.DB_USER,
+      password: process.env.DB_PASSWORD,
+      database: process.env.DB_NAME,
+      socketPath: process.env.DB_SOCKET
+    },
+    migrations: {
+      directory: './db/migrations'
+    },
+    seeds: {
+      directory: './db/seeds'
+    }
+  },
+
+  staging: {
+    client: 'mysql2',
+    connection: {
+      host: process.env.DB_HOST,
+      port: process.env.DB_PORT,
+      user: process.env.DB_USER,
+      password: process.env.DB_PASSWORD,
+      database: process.env.DB_NAME,
+      socketPath: process.env.DB_SOCKET
+    },
+    migrations: {
+      directory: './db/migrations'
+    },
+    seeds: {
+      directory: './db/seeds'
+    }
+  },
+
+  production: {
+    client: 'mysql2',
+    connection: {
+      host: process.env.DB_HOST,
+      port: process.env.DB_PORT,
+      user: process.env.DB_USER,
+      password: process.env.DB_PASSWORD,
+      database: process.env.DB_NAME,
+      socketPath: process.env.DB_SOCKET
+    },
+    migrations: {
+      directory: './db/migrations'
+    },
+    seeds: {
+      directory: './db/seeds'
+    }
+  }
+};