ci: add security and PHP validation workflow

[Emavero]

Summary

This PR introduces a new GitHub Actions workflow to provide automated security and code quality checks for the openDCIM project.

Features

Security Scanning

• Detect exposed secrets using Gitleaks
• Perform static application security testing (SAST) with Semgrep
• Scan dependencies and source code for known vulnerabilities using Trivy

PHP Validation

• Set up a PHP 8.2 environment
• Validate the composer.json configuration
• Install Composer dependencies
• Perform syntax validation on all PHP files

SBOM Generation

• Generate a Software Bill of Materials (SBOM)
• Submit dependency information to GitHub Dependency Graph

Benefits

• Detects PHP syntax errors early in the development process
• Helps identify security issues and exposed secrets
• Verifies Composer configuration and dependency installation
• Improves overall code quality and repository security
• Establishes a CI foundation that can be extended with unit and integration tests in the future

Motivation

The repository currently does not include a GitHub Actions workflow. This PR introduces an initial CI pipeline focused on security and PHP validation while remaining non-intrusive to the application's existing functionality.