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/app/logging_service.py

@@ -3,6 +3,7 @@ Logging Service for PVEmanager
 Provides centralized logging functionality for audit, system, and API events
 """
 
+import json
 import uuid
 import traceback
 from datetime import datetime, timedelta, timezone
@@ -74,7 +75,6 @@ class LoggingService:
                 return data[:max_size] + f"... (truncated, {len(data)} total chars)"
             return data
         if isinstance(data, dict):
-            import json
             json_str = json.dumps(data)
             if len(json_str) > max_size:
                 return {"_truncated": True, "_size": len(json_str), "_preview": json_str[:500]}