Support GitHub App authentication with client credentials

[robosek]

Currently, the server only supports authentication via Personal Access Tokens (PAT) or OAuth.

For organizations managing multiple integrations, GitHub Apps are preferred because they:

• Provide more granular permissions
• Include all repos in organization (PAT includes only 50)

Add support for GitHub App authentication using:

• App ID
• Client ID
• Private key (path to .pem file or secret)

This would allow the server to generate installation tokens programmatically instead of requiring PATs.

Alternative configuration:
{
"GITHUB_APP_ID": "123456",
"GITHUB_CLIENT_ID": "Iv1.abc123",
"GITHUB_PRIVATE_KEY_PATH": "/path/to/private-key.pem"
}

Is this on the roadmap?