Add Floating IP scanning and a durable address registry with configurable history depth

Adds POST /api/v1/admin/ips/scan (plus an optional periodic ticker) to
discover free Floating IPs in the OpenStack project and feed them straight
into the check queue. More importantly, decouples check/event history from
ip_queue's lifecycle: a new ip_registry table (migration 0007) gives every
address ever submitted a durable identity, so deleting it from the queue no
longer destroys its history — it's still reachable via the new
GET /api/v1/admin/registry[/{ip}] endpoints and the dashboard's /registry
pages, with retention depth configurable in check cycles per address
(history_retention_cycles, 0 = unlimited).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

internal/dashboard/templates/ip_detail.html

@@ -25,6 +25,7 @@
 <div class="topbar">
 <h1 class="mono" style="display:flex;align-items:center;gap:10px">{{.Detail.IP.IPAddress}} <span class="pill {{$b.Class}}">{{$b.Label}}</span></h1>
 </div>
+<p class="muted"><a href="/registry/{{.Detail.IP.IPAddress}}">Полная история проверок этого адреса за всё время &rarr;</a></p>
 <p class="muted">
 попытка {{.Detail.IP.AttemptNumber}} (повторов: {{.Detail.IP.RetryCount}})
 · валидатор {{deref .Detail.IP.OwnerValidatorID}}