feat: add new Prometheus metrics and bearer token auth for /metrics

Add 6 new Prometheus metrics for richer observability:
- auth_attempts_by_country_total (counter by country)
- commands_executed_total (counter by shell via OnCommand callback)
- human_score (histogram of final detection scores)
- storage_login_attempts_total, storage_unique_ips, storage_sessions_total
  (gauges via custom collector querying GetDashboardStats on each scrape)

Add optional bearer token authentication for the /metrics endpoint via
web.metrics_token config option. Uses crypto/subtle.ConstantTimeCompare.
Empty token (default) means no auth for backwards compatibility.

Also adds "cisco" to pre-initialized session/command metric labels.

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

internal/shell/shell.go

@@ -24,6 +24,7 @@ type SessionContext struct {
 	Store         storage.Store
 	ShellConfig   map[string]any
 	CommonConfig  ShellCommonConfig
+	OnCommand     func(shell string) // called when a command is executed; may be nil
 }
 
 // ShellCommonConfig holds settings shared across all shell types.