feat: add centralized error handler middleware

[Nishal77]

Summary

Adds a centralized Express error handler middleware that catches all unhandled errors and returns consistent JSON responses. Also fixes src/controller/error.ts which was using res.send() (plain text) instead of res.json(), causing client-side parse failures.

Related issue

Fixes #151

Type of change

• Bug fix
• New feature
• Documentation
• Chore / CI
• Breaking change

How to test

1. Checkout the branch
2. Run npm ci
3. Run npm start
4. In another terminal: curl -s http://localhost:9000/nonexistent | python3 -m json.tool
5. Verify HTTP 404 with JSON body: { "error": "fail", "message": "Can't find /nonexistent on this server!", "statusCode": 404 }
6. Verify health still works: curl -s http://localhost:9000/health | python3 -m json.tool

Checklist

• I have read the contributing guidelines
• I added tests that prove my fix is effective or that my feature works
• I updated documentation if necessary

Notes for reviewers

• src/middleware/errorHandler.ts — new file, named export errorHandler, four-param Express error handler (required for Express to recognize it as error middleware)
• src/index.ts — two lines added: import + app.use(errorHandler) after initializeAPI()
• src/controller/error.ts — res.send() changed to res.json() for consistent JSON shape across all errors

Release notes

Centralized error handler now returns consistent JSON { error, message, statusCode } for all unhandled errors.