security: fix critical vulnerabilities and code quality issues

Critical fixes:
- init_admin.py: replace hardcoded admin123 password with ADMIN_PASSWORD env var
- proxmox_client.py, models.py: change verify_ssl default from False to True
- api/proxmox.py: use server.verify_ssl instead of hardcoded verify=False in websocket handler
- ssh_client.py: replace AutoAddPolicy (MITM-vulnerable) with WarningPolicy + load_system_host_keys

Security hardening:
- main.py: add security headers middleware (CSP, X-Frame-Options, HSTS, X-XSS-Protection, Referrer-Policy)
- main.py: fix CORS — allow_credentials=True is incompatible with allow_origins=["*"]; read origins from CORS_ORIGINS env var
- auth.py, logging_middleware.py: validate X-Forwarded-For/X-Real-IP to prevent IP spoofing

Performance:
- monitoring_worker.py: wire up cleanup_expired_connections() as hourly scheduled job (was defined but never called)

Code quality:
- logging_service.py: move `import json` from inside function to module level
- ssh_client.py, proxmox_client.py, api/proxmox.py, rbac/migration.py, task_queue_service.py: replace all bare `except:` with `except Exception:`
- .env.example, config.py: document ADMIN_PASSWORD and CORS_ORIGINS settings

https://claude.ai/code/session_019rK2wvaWg5zXFwmAPPEB7R

backend/.env.example

@@ -26,8 +26,11 @@ DEFAULT_SSH_USER=root
 DEFAULT_SSH_PORT=22
 
 # Security
+# In production set to your actual domain(s), e.g.: https://panel.example.com
 CORS_ORIGINS=*
 ALLOWED_HOSTS=localhost,127.0.0.1
+# IMPORTANT: Change this before first deployment!
+ADMIN_PASSWORD=admin123
 
 # Update Check Configuration
 # Note: DISABLE_UPDATE_CHECK and GITHUB_TOKEN are managed via root .env