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>

oubliette.toml.example

@@ -43,6 +43,7 @@ retention_interval = "1h"
 # enabled = true
 # listen_addr = ":8080"
 # metrics_enabled = true
+# metrics_token = ""  # bearer token for /metrics; empty = no auth
 
 [shell]
 hostname = "ubuntu-server"