feat(event_bus): add in-process domain event bus

A standalone, domain-agnostic pub/sub seam: an EventBus that owns its
subscriber registry and streams Event values from producers to listeners
in process. Boundary-crossing (Celery/DB/workers) is left to subscribers,
keeping the bus single-responsibility. Includes the immutable Event value
object and full unit coverage.

surfsense_backend/app/event_bus/event.py

@@ -0,0 +1,38 @@
+"""The ``Event`` value object — the only shape that crosses the bus.
+
+An immutable fact: something named happened, with this payload, in this space,
+at this time. JSON round-trippable so a subscriber can queue it to a worker.
+"""
+
+from __future__ import annotations
+
+import uuid
+from datetime import UTC, datetime
+from typing import Any
+
+from pydantic import BaseModel, ConfigDict, Field
+
+
+def _new_event_id() -> str:
+    return uuid.uuid4().hex
+
+
+def _now() -> datetime:
+    return datetime.now(UTC)
+
+
+class Event(BaseModel):
+    """A published domain fact.
+
+    ``event_type`` is a dotted namespace (``document.indexed``, etc). ``payload`` is
+    JSON-serializable. ``search_space_id`` scopes delivery. ``event_id`` and
+    ``occurred_at`` are engine-stamped.
+    """
+
+    model_config = ConfigDict(frozen=True)
+
+    event_type: str
+    payload: dict[str, Any] = Field(default_factory=dict)
+    search_space_id: int
+    event_id: str = Field(default_factory=_new_event_id)
+    occurred_at: datetime = Field(default_factory=_now)