chore: document audit-log invalid-details fallback behavior

Agent-Logs-Url: https://github.com/bg-playground/BGSTM/sessions/3175915d-c82b-4f26-8b4e-82c07991f9dc

Co-authored-by: bg-playground <259109604+bg-playground@users.noreply.github.com>

Author: Copilot

backend/app/schemas/audit_log.py

@@ -32,6 +32,8 @@ def _normalize_details(cls, value: Any) -> dict[str, Any] | None:
             try:
                 parsed = json.loads(stripped)
             except json.JSONDecodeError:
+                # Preserve invalid historical payloads as structured data instead of
+                # failing audit-log reads for the entire response page.
                 return {"raw": value}
             if isinstance(parsed, dict):
                 return parsed