feat(storage): add SharePoint integration for document storage

Add Microsoft SharePoint Online as a storage provider using the
Microsoft Graph API. Includes OAuth2 authentication via MSAL,
site/drive resolution, chunked upload sessions, and metadata sync.

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

frontend/templates/help.html

@@ -33,7 +33,7 @@
       "name": "Which cloud storage providers does DocuElevate support?",
       "acceptedAnswer": {
         "@type": "Answer",
-        "text": "DocuElevate integrates with Dropbox, Google Drive, OneDrive, Amazon S3, Nextcloud, WebDAV, SFTP, FTP, and Paperless-ngx."
+        "text": "DocuElevate integrates with Dropbox, Google Drive, OneDrive, SharePoint, Amazon S3, Nextcloud, WebDAV, SFTP, FTP, and Paperless-ngx."
       }
     },
     {
@@ -196,6 +196,14 @@
         </div>
       </div>
 
+      <div class="flex items-start bg-white shadow rounded-lg p-5">
+        <i class="fab fa-microsoft text-purple-600 text-xl mr-3 mt-0.5" aria-hidden="true"></i>
+        <div>
+          <h3 class="font-semibold text-gray-800 text-sm">SharePoint</h3>
+          <p class="text-gray-500 text-xs">Upload to SharePoint Online document libraries via Graph API.</p>
+        </div>
+      </div>
+
       <div class="flex items-start bg-white shadow rounded-lg p-5">
         <i class="fab fa-aws text-yellow-600 text-xl mr-3 mt-0.5" aria-hidden="true"></i>
         <div>