feat(audit): M5.6 SourceNode sentinel backfill (purge-role) + CLI + runbook note (T5)

src/ZB.MOM.WW.ScadaBridge.CLI/ManagementHttpClient.cs

@@ -142,6 +142,60 @@ public class ManagementHttpClient : IDisposable
         return new ManagementResponse((int)httpResponse.StatusCode, null, error, code);
     }
 
+    /// <summary>
+    /// Issues a plain HTTP <c>POST</c> against a REST endpoint (e.g. the audit
+    /// maintenance endpoints) with a JSON body and returns the response. Unlike
+    /// <see cref="SendCommandAsync"/>, this does not wrap the call in the
+    /// <c>POST /management</c> command envelope — these are plain REST resources.
+    /// Authentication (HTTP Basic) and the base address are shared.
+    /// </summary>
+    /// <param name="relativePath">Path relative to the base URL.</param>
+    /// <param name="body">The JSON body to send, or <c>null</c> for an empty body.</param>
+    /// <param name="timeout">The request timeout.</param>
+    /// <returns>A management response containing status and data.</returns>
+    public async Task<ManagementResponse> SendPostAsync(string relativePath, string? body, TimeSpan timeout)
+    {
+        using var cts = new CancellationTokenSource(timeout);
+
+        var content = new StringContent(body ?? "{}", Encoding.UTF8, "application/json");
+
+        HttpResponseMessage httpResponse;
+        try
+        {
+            httpResponse = await _httpClient.PostAsync(relativePath, content, cts.Token);
+        }
+        catch (TaskCanceledException)
+        {
+            return new ManagementResponse(504, null, "Request timed out.", "TIMEOUT");
+        }
+        catch (HttpRequestException ex)
+        {
+            return new ManagementResponse(0, null, $"Connection failed: {ex.Message}", "CONNECTION_FAILED");
+        }
+
+        var responseBody = await httpResponse.Content.ReadAsStringAsync(cts.Token);
+
+        if (httpResponse.IsSuccessStatusCode)
+        {
+            return new ManagementResponse((int)httpResponse.StatusCode, responseBody, null, null);
+        }
+
+        string? error = null;
+        string? code = null;
+        try
+        {
+            using var doc = JsonDocument.Parse(responseBody);
+            error = doc.RootElement.TryGetProperty("error", out var e) ? e.GetString() : responseBody;
+            code = doc.RootElement.TryGetProperty("code", out var c) ? c.GetString() : null;
+        }
+        catch
+        {
+            error = responseBody;
+        }
+
+        return new ManagementResponse((int)httpResponse.StatusCode, null, error, code);
+    }
+
     /// <summary>
     /// Issues a plain HTTP <c>GET</c> and returns the raw <see cref="HttpResponseMessage"/>
     /// so the caller can stream the response body without buffering it in memory — used