fix(account): close deletion cache gap

Require authenticated sidecar cache purge before a deletion can complete and keep failures retryable. Mount tombstones outside restored application data while leaving deletion disabled by default.

JobTrackerApi/Services/AiSidecarCachePurger.cs

@@ -0,0 +1,16 @@
+namespace JobTrackerApi.Services;
+
+public interface IAiSidecarCachePurger
+{
+    Task PurgeAsync(CancellationToken cancellationToken);
+}
+
+public sealed class AiSidecarCachePurger(IHttpClientFactory clients) : IAiSidecarCachePurger
+{
+    public async Task PurgeAsync(CancellationToken cancellationToken)
+    {
+        using var response = await clients.CreateClient("ai-service")
+            .DeleteAsync("/maintenance/cache", cancellationToken);
+        response.EnsureSuccessStatusCode();
+    }
+}