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>

app/models.py

@@ -505,6 +505,7 @@ class IntegrationType:
     EMAIL = "EMAIL"
     PAPERLESS = "PAPERLESS"
     RCLONE = "RCLONE"
+    SHAREPOINT = "SHAREPOINT"
 
     ALL = {
         IMAP,
@@ -521,6 +522,7 @@ class IntegrationType:
         EMAIL,
         PAPERLESS,
         RCLONE,
+        SHAREPOINT,
     }