feat(ui): server-side streaming CSV export of Audit Log (#23 M7)

src/ScadaLink.CentralUI/Components/Pages/Audit/AuditLogPage.razor

@@ -19,6 +19,22 @@
                         InitialInstanceSearch="@_initialInstanceSearch" />
     </div>
 
+    @* Export button (Bundle F / M7-T14). A plain <a download> link triggers the
+       streaming CSV endpoint at /api/centralui/audit/export — chosen over a
+       SignalR-driven download because the request can stream 100k rows directly
+       to the response body without buffering through the Blazor circuit. The
+       href reflects the most recently applied filter; before Apply is clicked,
+       an unconstrained export is exposed. *@
+    <div class="mb-3 d-flex justify-content-end">
+        <a class="btn btn-outline-secondary btn-sm"
+           href="@ExportUrl"
+           download
+           role="button"
+           aria-label="Export current view to CSV">
+            Export CSV
+        </a>
+    </div>
+
     @* Results grid (Bundle B / M7-T3). Row clicks emit OnRowSelected for Bundle C's
        drilldown drawer; the grid stays in "no events" mode until the user applies a
        filter so the page does not auto-load the full audit table on first render. *@

src/ScadaLink.CentralUI/Components/Pages/Audit/AuditLogPage.razor.cs

@@ -1,3 +1,4 @@
+using System.Globalization;
 using Microsoft.AspNetCore.Components;
 using Microsoft.AspNetCore.WebUtilities;
 using ScadaLink.Commons.Entities.Audit;
@@ -158,4 +159,70 @@ public partial class AuditLogPage
         // grid) shows the same row instantly without a re-render flicker.
         _drawerOpen = false;
     }
+
+    /// <summary>
+    /// Bundle F (M7-T14): URL the Export-CSV link points at. Renders the most
+    /// recently applied filter as query-string params so the server-side
+    /// streaming endpoint reproduces the user's current view. With no filter
+    /// applied yet, returns the bare endpoint — i.e. an unconstrained export.
+    /// </summary>
+    /// <remarks>
+    /// Built here rather than in markup so the per-row test coverage can
+    /// exercise the URL composition without booting the full Blazor renderer.
+    /// </remarks>
+    internal string ExportUrl => BuildExportUrl(_currentFilter);
+
+    internal static string BuildExportUrl(AuditLogQueryFilter? filter)
+    {
+        const string basePath = "/api/centralui/audit/export";
+        if (filter is null)
+        {
+            return basePath;
+        }
+
+        var parts = new List<KeyValuePair<string, string?>>(9);
+        if (filter.Channel is { } ch)
+        {
+            parts.Add(new("channel", ch.ToString()));
+        }
+        if (filter.Kind is { } kind)
+        {
+            parts.Add(new("kind", kind.ToString()));
+        }
+        if (filter.Status is { } status)
+        {
+            parts.Add(new("status", status.ToString()));
+        }
+        if (!string.IsNullOrWhiteSpace(filter.SourceSiteId))
+        {
+            parts.Add(new("site", filter.SourceSiteId));
+        }
+        if (!string.IsNullOrWhiteSpace(filter.Target))
+        {
+            parts.Add(new("target", filter.Target));
+        }
+        if (!string.IsNullOrWhiteSpace(filter.Actor))
+        {
+            parts.Add(new("actor", filter.Actor));
+        }
+        if (filter.CorrelationId is { } corr)
+        {
+            parts.Add(new("correlationId", corr.ToString()));
+        }
+        if (filter.FromUtc is { } from)
+        {
+            parts.Add(new("from", from.ToString("O", CultureInfo.InvariantCulture)));
+        }
+        if (filter.ToUtc is { } to)
+        {
+            parts.Add(new("to", to.ToString("O", CultureInfo.InvariantCulture)));
+        }
+
+        if (parts.Count == 0)
+        {
+            return basePath;
+        }
+
+        return QueryHelpers.AddQueryString(basePath, parts);
+    }
 }