fix: address Task 6 quality review — rename helper, restore 403 for intra-account, add docs test

- Rename _get_tree_or_403 → _get_tree_or_404 in maintenance_schedules.py
  (function now raises 404, old name was misleading)
- Restore HTTP 403 for intra-account permission failures in update_tree:
  same-account users who can see a tree but can't edit it got 404 (wrong);
  only cross-account lookups should return 404 to avoid confirming existence
- Apply same 403/404 distinction to update_tree_visibility
- Add test: get_documentation must return 404 for cross-user session access
- Add comment documenting owner-only design for documentation endpoints

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

backend/app/api/endpoints/trees.py

@@ -610,6 +610,14 @@ async def update_tree(
         )
 
     if not can_edit_tree(current_user, tree):
+        # If the user can see this tree (same account, team visibility), give a 403 with
+        # a clear message — returning 404 here would be confusing since GET returns 200.
+        # For truly inaccessible trees (cross-account), return 404 to avoid confirming existence.
+        if can_access_tree(current_user, tree):
+            raise HTTPException(
+                status_code=status.HTTP_403_FORBIDDEN,
+                detail="You do not have permission to edit this flow"
+            )
         raise HTTPException(
             status_code=status.HTTP_404_NOT_FOUND,
             detail="Tree not found"
@@ -1144,6 +1152,14 @@ async def update_tree_visibility(
         )
 
     if not can_edit_tree(current_user, tree):
+        # If the user can see this tree (same account, team visibility), give a 403 with
+        # a clear message — returning 404 here would be confusing since GET returns 200.
+        # For truly inaccessible trees (cross-account), return 404 to avoid confirming existence.
+        if can_access_tree(current_user, tree):
+            raise HTTPException(
+                status_code=status.HTTP_403_FORBIDDEN,
+                detail="You do not have permission to edit this flow"
+            )
         raise HTTPException(
             status_code=status.HTTP_404_NOT_FOUND,
             detail="Tree not found"