feat: update course entity & refactor package

src/main/resources/db/migration/V1__course_module.sql

@@ -0,0 +1,8 @@
+-- Flyway Migration V1: Initial schema
+-- Creates courses table
+
+CREATE TABLE IF NOT EXISTS courses (
+    id BIGSERIAL PRIMARY KEY,
+    title VARCHAR(200) NOT NULL,
+    description VARCHAR(1000)
+);