Wire auto-elevate ownership across all SharePoint operations

The "Auto-elevate ownership when permission scan is denied" setting was
dead code: the toggle was persisted but never read, the audit flow never
passed its onAccessDenied callback, and EnrichException wrapped every CSOM
error (including ServerUnauthorizedAccessException) into a generic
InvalidOperationException so the access-denied catch could never match.

Centralize elevation instead of per-call-site callbacks:

- Throw typed SharePointAccessDeniedException from EnrichException on
  access-denied, preserving the failing site URL and enriched diagnostic.
- Add scoped IElevationCoordinator that catches it, and when AutoTakeOwnership
  is enabled takes site-collection admin via the tenant admin endpoint and
  retries the operation once. Per-site dedupe prevents loops; admin-host
  denials are not treated as ownership issues. Retry is safe because each
  wrapped operation closure re-issues its own CSOM loads.
- Wrap all site-scoped operations (Storage, Permissions, Duplicates, Search,
  VersionCleanup, FolderStructure, BulkMembers, FileTransfer, Templates) and
  the UserAccessAudit per-site scan in the coordinator.
- Drop the unused onAccessDenied parameter from IUserAccessAuditService.

Elevation still requires SharePoint tenant admin rights on the signed-in
account; the coordinator surfaces a clear message when that is missing.

Also keeps the prior StorageService change that avoids admin-gated
folder.StorageMetrics (403 for delegated non-admin tokens).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Core/Helpers/SharePointAccessDeniedException.cs

@@ -0,0 +1,18 @@
+namespace SharepointToolbox.Web.Core.Helpers;
+
+/// <summary>
+/// Thrown when a CSOM operation fails with a SharePoint "access denied"
+/// (System.UnauthorizedAccessException / ServerUnauthorizedAccessException).
+/// Carries the failing site URL so the elevation coordinator can take site-collection
+/// admin ownership and retry. Message is the enriched diagnostic from EnrichException.
+/// </summary>
+public sealed class SharePointAccessDeniedException : Exception
+{
+    public string SiteUrl { get; }
+
+    public SharePointAccessDeniedException(string message, string siteUrl, Exception inner)
+        : base(message, inner)
+    {
+        SiteUrl = siteUrl;
+    }
+}