feat(ui): drill-ins from detail pages to Audit Log (#23 M7)

Adds "Recent audit activity" deep links from four edit/detail pages into
the central Audit Log, each with a pre-filter encoded in the query string
that the Audit Log page (Bundle D0) now parses on initialization:

  - External Systems (Design/ExternalSystemForm)      → ?target={Name}
  - API Keys         (Admin/ApiKeyForm)                → ?actor={Name}&channel=ApiInbound
  - Sites            (Admin/SiteForm)                  → ?site={SiteIdentifier}
  - Instances        (Deployment/InstanceConfigure)    → ?instance={UniqueName}

The link is suppressed on create/new flows where there is nothing to
drill into yet. Instance is UI-only on the filter bar (the repository
filter contract has no instance column), so the page-side prefill threads
through the InitialInstanceSearch seam on AuditFilterBar.

Site Calls (#22 M7-T11) drill-in is DEFERRED: the Central UI does not
yet host a Site Calls listing page, per M3 reality notes. Add the
drill-in when that page lands.

#23 M7-T12

src/ScadaLink.CentralUI/Components/Pages/Design/ExternalSystemForm.razor

@@ -10,7 +10,20 @@
 <div class="container-fluid mt-3">
     <button class="btn btn-link text-decoration-none ps-0 mb-2" @onclick="GoBack">&larr; Back</button>
 
-    <h4 class="mb-3">@(Id.HasValue ? "Edit External System" : "Add External System")</h4>
+    <div class="d-flex justify-content-between align-items-center mb-3">
+        <h4 class="mb-0">@(Id.HasValue ? "Edit External System" : "Add External System")</h4>
+        @* Bundle D (#23 M7-T12) drill-in: deep-link into the central Audit Log
+           pre-filtered to this external system's outbound API events. Audit rows
+           record the target by external-system name, so we filter on Target. *@
+        @if (Id.HasValue && !string.IsNullOrWhiteSpace(_name))
+        {
+            <a class="btn btn-outline-secondary btn-sm"
+               href="/audit/log?target=@Uri.EscapeDataString(_name)"
+               data-test="audit-link">
+                Recent audit activity
+            </a>
+        }
+    </div>
 
     @if (_loading)
     {