security: Add request body size limits to API handlers
Add http.MaxBytesReader to 16 additional handlers to prevent memory
exhaustion attacks via oversized request bodies:
- docker_agents.go: HandleReport (512KB), HandleCommandAck (8KB),
HandleSetCustomDisplayName (8KB)
- alerts.go: UpdateAlertConfig (64KB), BulkAcknowledgeAlerts (32KB),
BulkClearAlerts (32KB)
- config_handlers.go: HandleAddNode, HandleTestConnection,
HandleUpdateNode, HandleTestNodeConfig (32KB each),
HandleVerifyTemperatureSSH, HandleExportConfig, HandleDiscoverServers,
HandleSetupScriptURL (8KB each), HandleImportConfig (1MB),
HandleUpdateMockMode (16KB)