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/handlers_ips.go

@@ -117,3 +117,11 @@ func (s *Server) handleIPsClear(w http.ResponseWriter, r *http.Request) {
 	_, err := s.CA.ClearQueue(r.Context())
 	s.renderIPsTable(w, r, err)
 }
+
+// handleIPsScan lists the OpenStack project's free (unassociated) floating
+// IPs and submits them to the check queue in one step — see
+// client.ScanFloatingIPs.
+func (s *Server) handleIPsScan(w http.ResponseWriter, r *http.Request) {
+	_, err := s.CA.ScanFloatingIPs(r.Context())
+	s.renderIPsTable(w, r, err)
+}