test(backend): pin the traversal guard against a string-prefix bypass

Review round 1: the two existing safe_abs_path escape tests both pass even against
the classic buggy `str(path).startswith(str(root))` containment check, so a
regression to that bug would have shipped green. Add the one input that
distinguishes the correct `root not in path.parents` guard: a sibling dir whose name
prefixes the root (/root vs /rootx). Mutation-verified — swapping the guard to
startswith now fails this test. Also corrected the requirements-dev header (installed
by Dockerfile.test, not a nonexistent `dev` stage).

backend/tests/test_storage.py

@@ -91,5 +91,17 @@ class TestSafeAbsPath:
         outside.write_text("nope")
         assert safe_abs_path(str(root), str(outside)) is None
 
+    def test_rejects_a_sibling_dir_whose_name_prefixes_the_root(self, tmp_path: Path):
+        # The one input that distinguishes the correct `root not in path.parents` guard from the
+        # classic buggy `str(path).startswith(str(root))`: /root vs a real sibling /rootx. A file in
+        # rootx is OUTSIDE root, but its path DOES start with the root string — a prefix check would
+        # wrongly serve it. This pins the guard against that regression.
+        root = tmp_path / "root"
+        root.mkdir()
+        sibling = tmp_path / "rootx"
+        sibling.mkdir()
+        (sibling / "steal.mp4").write_text("nope")
+        assert safe_abs_path(str(root), "../rootx/steal.mp4") is None
+
     def test_returns_none_for_a_nonexistent_file_inside_the_root(self, tmp_path: Path):
         assert safe_abs_path(str(tmp_path), "does/not/exist.mp4") is None