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/config/config.go

@@ -21,9 +21,10 @@ type Config struct {
 }
 
 type WebConfig struct {
-	Enabled        bool  `toml:"enabled"`
+	Enabled        bool   `toml:"enabled"`
 	ListenAddr     string `toml:"listen_addr"`
-	MetricsEnabled *bool `toml:"metrics_enabled"`
+	MetricsEnabled *bool  `toml:"metrics_enabled"`
+	MetricsToken   string `toml:"metrics_token"`
 }
 
 type ShellConfig struct {