fix: return 404 instead of 403 for cross-team tree access in maintenance schedules

_get_tree_or_403 now returns 404 when the user's team does not match,
preventing confirmation of tree existence across teams.

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

backend/app/api/endpoints/maintenance_schedules.py

@@ -38,7 +38,7 @@ async def _get_tree_or_403(tree_id: UUID, current_user: User, db: AsyncSession)
     # Super admins can access any tree; regular users must be on the same team
     if not getattr(current_user, 'is_super_admin', False):
         if tree.team_id != current_user.team_id:
-            raise HTTPException(status_code=403, detail="Access denied")
+            raise HTTPException(status_code=404, detail="Tree not found")
     return tree