Migrate from DynamoDB to RDS

[Douglas-Hong]

Summary

• Created a backend migrate script to get user IDs from DynamoDB and then insert users to Postgres.
• We still have to fill in more information for each user. Currently, I just insert user IDs. Schedules and other information still have to be migrated from Dynamo to Postgres.
• Before migrating, set up AWS CLI credentials, make sure you have the correct DynamoDB table name in your .env file, run pnpm run generate to generate the migration files, and make sure the Postgres DB specified in apps/backend/src/db/index.ts exists. Then, run pnpm run migrate to execute the migration.