fix(v2): enforce repository safety failures

backend/src/backup_tool/repository.py

@@ -29,10 +29,19 @@ def blob_digest(content: bytes) -> str:
 
 
 def _contained(root: Path, relative_path: str) -> Path:
-    candidate = (root / relative_path).resolve()
-    if not relative_path or Path(relative_path).is_absolute() or candidate == root:
+    relative = Path(relative_path)
+    if not relative_path or relative.is_absolute():
         raise RepositoryError("repository path must be a non-empty relative child")
-    if root not in candidate.parents:
+    lexical = root
+    for part in relative.parts:
+        lexical /= part
+        if lexical.is_symlink():
+            raise RepositoryError("repository path contains a symlink")
+    candidate = lexical.resolve()
+    resolved_root = root.resolve()
+    if candidate == resolved_root:
+        raise RepositoryError("repository path must be a non-empty relative child")
+    if resolved_root not in candidate.parents:
         raise RepositoryError("repository path escapes configured roots")
     return candidate