[BACKEND] Add pagination to `GET /transactions`

[Trovic1]

Summary

Support ?page= and ?limit= query params on the transactions endpoint, returning page metadata.

Scope

• backend only

Likely files

• src/routes/TransactionRoutes.js
• src/models/Transaction.js

Acceptance criteria

• Accepts page and limit with sane defaults and caps
• Response includes total count and current page
• Works in both Postgres and local-JSON modes

Verification

• Seed many transactions and confirm paging returns correct slices

Labels: backend, enhancement, help wanted

---