fix: improve error classification for better observability (#618)

[rajprakhar07]

Closes #618

What I did

• Added ErrorCategory enum (CLIENT, SERVER, PROVIDER)
• Added ERROR_CATEGORY_MAP to map HTTP status codes to categories
• Added error_classification_middleware that attaches X-Error-Category header to all error responses
• Registered middleware in main.py
• Errors are now logged with category for better observability and triage

Files Changed

• backend/app/middleware.py — added ErrorCategory enum and middleware
• backend/app/main.py — registered error_classification_middleware

Type

• Bug fix / Enhancement

[rajprakhar07]

Tested the implementation locally:
curl -i http://127.0.0.1:8000/nonexistent-route
HTTP/1.1 404 Not Found
x-error-category: client_error
Middleware is working correctly — errors are classified and X-Error-Category header is attached to all error responses. Kindly review and merge!