feat(gateway): add gateway domain primitives

surfsense_backend/app/gateway/base/identity.py

@@ -0,0 +1,19 @@
+"""Gateway identity helpers."""
+
+from __future__ import annotations
+
+import hashlib
+
+
+def normalize_external_peer_id(value: str | int | None) -> str | None:
+    if value is None:
+        return None
+    return str(value).strip()
+
+
+def hash_external_id(value: str | int | None) -> str | None:
+    normalized = normalize_external_peer_id(value)
+    if not normalized:
+        return None
+    return hashlib.sha256(normalized.encode("utf-8")).hexdigest()
+