Improve validation and robustness of GitHub webhook handler

## Summary
While exploring the backend architecture, I reviewed the `/github/webhook` endpoint defined in `backend/routes.py`. Currently, the webhook payload is accepted and processed without explicit schema validation or structured error handling for malformed or unexpected event data.

## Problem
The handler directly parses the incoming JSON payload and maps events based on headers and action fields. If the payload structure changes or contains missing keys, it may lead to silent failures or unhandled edge cases.

This could affect reliability when processing GitHub events such as issues, pull requests, and comments.

## Proposed Improvement
I propose the following enhancements:
- Add schema validation for incoming webhook payloads (using Pydantic models)
- Improve error handling for missing or malformed fields
- Add more structured logging for unsupported or unknown event types
- Ensure graceful handling of unexpected payload formats

## Benefits
- Increased robustness and reliability of GitHub event processing
- Better debugging and observability through structured logs
- Safer handling of webhook payload variations from GitHub

## Next Steps
If this approach aligns with the project direction, I would be happy to work on implementing these improvements and submit a pull request.