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/cisco/cisco.go

@@ -74,6 +74,9 @@ func (c *CiscoShell) Handle(ctx context.Context, sess *shell.SessionContext, rw
 					return fmt.Errorf("append session log: %w", err)
 				}
 			}
+			if sess.OnCommand != nil {
+				sess.OnCommand("cisco")
+			}
 			continue
 		}
 
@@ -92,6 +95,9 @@ func (c *CiscoShell) Handle(ctx context.Context, sess *shell.SessionContext, rw
 				return fmt.Errorf("append session log: %w", err)
 			}
 		}
+		if sess.OnCommand != nil {
+			sess.OnCommand("cisco")
+		}
 
 		if result.exit {
 			return nil