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>

rxprod-compose/sources/control-api.example.yaml

@@ -54,6 +54,12 @@ orchestrator:
   # /settings page) and this field is ignored. Must satisfy
   # fip_settle_seconds + self_check_timeout_seconds < lease_ttl_seconds.
   fip_settle_seconds: 0
+  # How often (seconds) to automatically scan the OpenStack project for free
+  # (unassociated) floating IPs and submit them to the check queue. 0 (the
+  # default) disables periodic scanning — an operator can still trigger a
+  # scan on demand via POST /api/v1/admin/ips/scan or the dashboard's
+  # "Scan Floating IPs" button.
+  fip_scan_interval_seconds: 0
 
 aggregation:
   missing_counts_as_fail: true