Unify workspace root resolution across services

backend/services/workspace_paths.py

@@ -0,0 +1,19 @@
+"""Shared workspace path helpers.
+
+Single authority for workspace root and per-user workspace paths.
+"""
+
+from pathlib import Path
+
+from utils.storage_paths import get_repo_root, sanitize_user_id
+
+
+def get_workspace_root() -> Path:
+    """Return absolute workspace root directory under repo root."""
+    return (get_repo_root() / "workspace").resolve()
+
+
+def get_user_workspace_dir(user_id: str) -> Path:
+    """Return absolute workspace directory for the given user."""
+    safe_user_id = sanitize_user_id(user_id)
+    return (get_workspace_root() / f"workspace_{safe_user_id}").resolve()