feat: add optional basic auth support for Loki client

Some Loki deployments (e.g., behind a reverse proxy or Grafana Cloud)
require HTTP Basic Authentication. This adds optional --loki-username
and --loki-password flags (and corresponding env vars) to the
lab-monitoring server, along with NixOS module options for secure
credential management via systemd LoadCredential.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/mcp/server.go

@@ -87,7 +87,7 @@ This ensures package information matches the nixpkgs version the project actuall
 func DefaultMonitoringConfig() ServerConfig {
 	return ServerConfig{
 		Name:    "lab-monitoring",
-		Version: "0.3.0",
+		Version: "0.3.1",
 		Mode:    ModeCustom,
 		Instructions: `Lab Monitoring MCP Server - Query Prometheus metrics and Alertmanager alerts.