fix(membership): correct \$unwind option preserveNullAndEmpty → preserveNullAndEmptyArrays

MongoDB 7.0 rejects the invalid key with code 28811, causing 500 on
GET /organizations/{id}/members.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/services/membership_service.py

@@ -109,7 +109,7 @@ async def list_org_members(
                 "as": "user_doc",
             }
         },
-        {"$unwind": {"path": "$user_doc", "preserveNullAndEmpty": False}},
+        {"$unwind": {"path": "$user_doc", "preserveNullAndEmptyArrays": False}},
         {"$sort": {"created_at": 1}},
     ]
     details = []