feat(gateway): add gateway domain primitives

surfsense_backend/app/gateway/base/translator.py

@@ -0,0 +1,28 @@
+"""Base stream translator for platform-specific outbound UX."""
+
+from __future__ import annotations
+
+from abc import ABC, abstractmethod
+from collections.abc import AsyncIterator
+from dataclasses import dataclass, field
+from typing import Any
+
+
+@dataclass(frozen=True)
+class GatewayStreamEvent:
+    """Small provider-neutral event shape consumed by translators.
+
+    The existing chat stack emits Vercel/assistant-ui events.  Gateway code
+    normalizes the subset it needs into this shape before handing it to the
+    platform translator.
+    """
+
+    type: str
+    data: dict[str, Any] = field(default_factory=dict)
+
+
+class BaseStreamTranslator(ABC):
+    @abstractmethod
+    async def translate(self, events: AsyncIterator[GatewayStreamEvent]) -> None:
+        """Consume agent stream events and emit platform messages."""
+