Implement GitHub webhook signature verification

[TooAngel]

Add webhook signature verification for enhanced security.

Context:

• Current webhook endpoint at /github processes events without signature verification
• GitHub provides webhook signatures for security validation
• Enhanced security measure for production environment

Acceptance Criteria:

• Implement webhook signature verification using GitHub secret
• Configure webhook secret in GitHub repository settings
• Add environment variable for webhook secret
• Test signature verification with actual webhook events
• Document webhook security setup

Priority: Medium