[MSFNW26] Setup Sequlize

[tonyqiu123]

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Implementation description

• Added Sequelize with TypeScript support (sequelize-typescript, pg) as the backend ORM for PostgreSQL.
• Set up Umzug for TypeScript migrations with SequelizeStorage; migrations live in migrations/ and are run via migrate.js.
• Added .sequelizerc for config/models/seeders/migrations paths; umzug.ts wires Sequelize (dev: POSTGRES_*, DB_HOST, POSTGRES_DB_DEV; prod: DATABASE_URL).
• No new routes or models in this PR—setup only for future migrations and models.

Steps to test

1. Start Postgres: docker-compose up -d db
2. In backend/, add a .env with POSTGRES_USER, POSTGRES_PASSWORD, DB_HOST=db, POSTGRES_DB_DEV=mfsn_db
3. Run cd backend && npm install then node migrate.js up (should succeed; no migrations yet is fine)
4. Optionally run node migrate.js create --name example-migration.ts --folder migrations and node migrate.js up again to confirm migration creation

What should reviewers focus on?

• Whether to add a .env.example or README note for DB_HOST and POSTGRES_DB_DEV
• That umzug.ts path resolution from backend/ is correct when running node migrate.js

Checklist

• My PR name is descriptive and in imperative tense
• My commit messages are descriptive and in imperative tense. My commits are atomic and trivial commits are squashed or fixup'd into non-trivial commits
• I have run the appropriate linter(s)
• I have requested a review from the PL, as well as other devs who have background knowledge on this PR or who will be building on top of this PR