feat: add validation to ensure event registrations do not exceed capa…

[mug1sha]

##What does this PR do?

This PR implements validation guards for event creation and updates to ensure that the registered count does not exceed the event's capacity. It handles both new input data and
cross-field checks against existing database values during partial updates to maintain business logic integrity.

##Related issue

Closes #28

##How did you test it?

• Unit Tests: Created src/controllers/event.controller.test.ts with test cases verifying that exceeding capacity returns a 400 Bad Request in both POST /api/events and PUT
  /api/events/:id scenarios.
• Integration: Verified that the entire test suite passes (npm test), confirming no regressions in existing event functionality.
• Quality Checks: Confirmed the code follows project standards by running npm run lint and npm run format.

##Checklist

• My code builds (npm run build)
• I added or updated tests and npm test passes
• I updated docs/openapi.yaml if I changed any routes
• I added a Prisma migration if I changed schema.prisma
• I updated docs or .env.example if needed (Updated README.md with capacity validation details)