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_registry.go

@@ -0,0 +1,37 @@
+package dashboard
+
+import "net/http"
+
+type registryPageData struct {
+	PageData
+	Items []registryItem
+}
+
+type registryDetailData struct {
+	PageData
+	History registryHistoryResponse
+}
+
+// handleRegistryPage lists every address ever submitted to the check
+// queue, with a summary of its accumulated check history — the durable
+// record that survives an address being deleted from /ips and later
+// re-added. See internal/db/migrations/0007_ip_registry.sql.
+func (s *Server) handleRegistryPage(w http.ResponseWriter, r *http.Request) {
+	items, err := s.CA.ListRegistry(r.Context())
+	data := registryPageData{Items: items}
+	data.ActiveNav = "registry"
+	data.Banner = bannerFor(err)
+	s.renderPage(w, "registry_page", data)
+}
+
+// handleRegistryDetail shows one address's full retained check history
+// across every cycle it has ever run, not just the current attempt — see
+// ip_detail_content in ip_detail.html for the attempt-scoped equivalent.
+func (s *Server) handleRegistryDetail(w http.ResponseWriter, r *http.Request) {
+	ip := r.PathValue("ip")
+	history, err := s.CA.GetRegistryHistory(r.Context(), ip)
+	data := registryDetailData{History: history}
+	data.ActiveNav = "registry"
+	data.Banner = bannerFor(err)
+	s.renderPage(w, "registry_detail_page", data)
+}