✨(backend) add "tree" action on document API endpoint

We want to display the tree structure to which a document belongs
on the left side panel of its detail view. For this, we need an
endpoint to retrieve the list view of the document's ancestors
opened.

By opened, we mean that when display the document, we also need to
display its siblings. When displaying the parent of the current
document, we also need to display the siblings of the parent...

src/backend/core/api/utils.py

@@ -11,6 +11,35 @@ import botocore
 from rest_framework.throttling import BaseThrottle
 
 
+def nest_tree(flat_list, steplen):
+    """
+    Convert a flat list of serialized documents into a nested tree making advantage
+    of the`path` field and its step length.
+    """
+    node_dict = {}
+    roots = []
+
+    # Sort the flat list by path to ensure parent nodes are processed first
+    flat_list.sort(key=lambda x: x["path"])
+
+    for node in flat_list:
+        node["children"] = []  # Initialize children list
+        node_dict[node["path"]] = node
+
+        # Determine parent path
+        parent_path = node["path"][:-steplen]
+
+        if parent_path in node_dict:
+            node_dict[parent_path]["children"].append(node)
+        else:
+            roots.append(node)  # Collect root nodes
+
+    if len(roots) > 1:
+        raise ValueError("More than one root element detected.")
+
+    return roots[0] if roots else None
+
+
 def filter_root_paths(paths, skip_sorting=False):
     """
     Filters root paths from a list of paths representing a tree structure.