feat: add API key authentication system for ChatGPT Business

Implements comprehensive Bearer token authentication to ensure only
authorized ChatGPT workspaces can access the MCP server.

Core Features:
- API key validation with constant-time comparison
- Multi-key support for rotation grace periods
- Rate limiting (5 failures per IP per 5 min)
- Comprehensive audit logging of all auth attempts
- IP-based failed attempt tracking

Key Management:
- generate_api_key.py: Create secure 64-char keys
- rotate_api_key.py: Guided key rotation with backup
- check_key_age.py: Automated expiration monitoring

Infrastructure:
- Traefik labels for HTTPS and rate limiting
- Security headers (HSTS, CSP, X-Frame-Options)
- Environment-based configuration
- Docker secrets support

Documentation:
- AUTH_SETUP.md: Complete authentication setup guide
- CHATGPT_SETUP.md: ChatGPT Business integration guide
- KEY_ROTATION.md: Key rotation procedures and automation

Security:
- Read-only operations enforced
- No write access to Gitea possible
- All auth attempts logged with correlation IDs
- Failed attempts trigger IP rate limits
- Keys never logged in full (only hints)

Breaking Changes:
- AUTH_ENABLED defaults to true
- MCP_API_KEYS environment variable now required
- Minimum key length: 32 characters (64 recommended)

Migration:
1. Generate API key: make generate-key
2. Add to .env: MCP_API_KEYS=<generated-key>
3. Restart: docker-compose restart aegis-mcp
4. Configure ChatGPT with Authorization header

Closes requirements for ChatGPT Business exclusive access.

.gitignore

@@ -44,6 +44,11 @@ htmlcov/
 # Environment variables
 .env
 .env.local
+.env.backup-*
+
+# API keys
+keys/
+*.key
 
 # Logs
 *.log