Add streaming emitter and registry for scoped SSE writes.

surfsense_backend/app/services/streaming/emitter/registry.py

@@ -0,0 +1,51 @@
+"""Resolve which agent owns a streamed event from its LangGraph run lineage."""
+
+from __future__ import annotations
+
+from collections.abc import Iterable
+
+from .emitter import Emitter, main_emitter
+
+
+class EmitterRegistry:
+    def __init__(self) -> None:
+        self._by_run_id: dict[str, Emitter] = {}
+
+    def register(self, run_id: str, emitter: Emitter) -> None:
+        if not run_id:
+            return
+        self._by_run_id[run_id] = emitter
+
+    def unregister(self, run_id: str) -> Emitter | None:
+        if not run_id:
+            return None
+        return self._by_run_id.pop(run_id, None)
+
+    def get(self, run_id: str | None) -> Emitter | None:
+        if not run_id:
+            return None
+        return self._by_run_id.get(run_id)
+
+    def resolve(
+        self,
+        *,
+        run_id: str | None,
+        parent_ids: Iterable[str] | None,
+    ) -> Emitter:
+        own = self.get(run_id)
+        if own is not None:
+            return own
+        if parent_ids:
+            for ancestor in reversed(list(parent_ids)):
+                emitter = self.get(ancestor)
+                if emitter is not None:
+                    return emitter
+        return main_emitter()
+
+    def has_active_subagents(self) -> bool:
+        return any(
+            emitter.level == "subagent" for emitter in self._by_run_id.values()
+        )
+
+    def clear(self) -> None:
+        self._by_run_id.clear()