feat(api) add consistent error codes for API responses

[Krishnavamsi-codes]

Description

This PR implements consistent error codes for API responses across the backend.

Changes made

• Added centralized exception handling via backend/app/exceptions.py

• Introduced an ErrorCode enum with standardized string-based error codes

• Added an ErrorResponse schema for consistent API error responses

• Registered global handlers for:

  • HTTPException
  • RequestValidationError
  • Unhandled exceptions

• Updated middleware responses to include error codes

• Standardized API error responses to the format:

{
  "error": "error_code",
  "detail": "Human-readable message"
}

• Preserved existing HTTP status codes and detail messages
• Updated backend tests to validate the new error response schema

Related Issue

Fixes #509

Type of change

• Bug fix
• New feature / enhancement
• Documentation update
• Test addition
• Refactor

Checklist

• I have read CONTRIBUTING.md
• My branch is up to date with main
• I have run pytest -v and all tests pass
• I have not introduced duplicate issues or features
• My PR title follows the format: feat/fix/docs/test: short description
• I have added tests for new features (Level 2 and 3 issues)
• No hardcoded secrets or API keys in my code
• This PR is linked to a GSSoC 2026 issue

Screenshots (if frontend change)

N/A

Test evidence

pytest -v

==================== test session starts ====================
372 passed, 9 warnings in 17.98s
==================== test session finished ==================