Add vulnerability scanner and WebSocket manager for scan notifications

- Implemented VulnerabilityScanner class to scan images for vulnerabilities using Trivy and NVD API.
- Added methods to parse and store vulnerability data in the database.
- Created WebSocketManager class to handle real-time notifications for scan status updates.
- Integrated WebSocket notifications for scan start, completion, and failure events.

.env.template

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+# GitLab Configuration
+# Required: Your GitLab personal access token with API access
+GITLAB_TOKEN=gitlab-token
+
+# Optional: GitLab instance URL (defaults to https://gitlab.com)
+GITLAB_URL=https://gitlab.popei.fr
+
+# Optional: Comma-separated list of GitLab group IDs or paths to scan (if not set, scans all projects)
+# Examples: GITLAB_GROUPS=my-group,another-group or GITLAB_GROUPS=123,456
+# GITLAB_GROUPS=
+
+# Database Configuration
+# Optional: SQLite database file path (defaults to ./gitlab_docker_tracker.db)
+DATABASE_URL=sqlite:///./gitlab_docker_tracker.db
+
+# API Configuration
+# Optional: FastAPI host (defaults to 0.0.0.0)
+API_HOST=0.0.0.0
+
+# Optional: FastAPI port (defaults to 5000)
+API_PORT=5000
+
+# Optional: Enable debug mode (defaults to false)
+DEBUG=true
+
+# Vulnerability Scanning Configuration
+# Optional: Trivy binary path (auto-detected if available)
+# TRIVY_PATH=/usr/local/bin/trivy
+
+# Optional: CVE API base URL (defaults to NVD API)
+# CVE_API_BASE=https://services.nvd.nist.gov/rest/json/cves/2.0
+
+# Optional: Request timeout for external APIs in seconds (defaults to 30)
+# REQUEST_TIMEOUT=30
+
+# Scanning Configuration
+# Optional: Maximum number of repositories to scan in parallel (defaults to 5)
+# MAX_CONCURRENT_SCANS=5
+
+# Optional: Scan timeout per repository in seconds (defaults to 300)
+# SCAN_TIMEOUT=300
+
+# Optional: Branches to scan (comma-separated, defaults to main,master,develop)
+# TARGET_BRANCHES=main,master,develop
+
+# Logging Configuration
+# Optional: Log level (DEBUG, INFO, WARNING, ERROR, CRITICAL)
+LOG_LEVEL=DEBUG
+
+# Optional: Log file path (if not set, logs to stdout)
+# LOG_FILE=/var/log/gitlab-docker-tracker.log