feat: add createdAt field to Todo entity and update database migration script

backend/src/main/resources/db/migration/V1__init.sql

@@ -0,0 +1,7 @@
+CREATE TABLE todos (
+    id BIGSERIAL PRIMARY KEY,
+    title VARCHAR(255) NOT NULL,
+    description TEXT,
+    completed BOOLEAN NOT NULL DEFAULT FALSE,
+    created_at TIMESTAMP WITH TIME ZONE NOT NULL DEFAULT NOW()
+);