refactor(gateway): move sanitize_log_param to app/gateway/utils.py

Extract the log-injection sanitizer from routers/threads.py into a shared
utils module and rename to sanitize_log_param (public API). Eliminates the
reverse service → router import in services.py.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/gateway/utils.py

@@ -0,0 +1,6 @@
+"""Shared utility helpers for the Gateway layer."""
+
+
+def sanitize_log_param(value: str) -> str:
+    """Strip control characters to prevent log injection."""
+    return value.replace("\n", "").replace("\r", "").replace("\x00", "")