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/tests/unit/event_bus/test_event.py

@@ -0,0 +1,53 @@
+"""``Event`` contract: carry caller facts + engine-stamped id/time, round-trip JSON."""
+
+from __future__ import annotations
+
+from datetime import datetime
+
+import pytest
+
+from app.event_bus.event import Event
+
+pytestmark = pytest.mark.unit
+
+
+def test_event_carries_caller_supplied_facts() -> None:
+    """The three caller inputs are stored verbatim."""
+    event = Event(
+        event_type="document.indexed",
+        payload={"document_id": 42, "content_type": "pdf"},
+        search_space_id=7,
+    )
+
+    assert event.event_type == "document.indexed"
+    assert event.payload == {"document_id": 42, "content_type": "pdf"}
+    assert event.search_space_id == 7
+
+
+def test_event_stamps_identity_and_time_when_not_supplied() -> None:
+    """Engine stamps id + time so subscribers can dedup/order."""
+    event = Event(event_type="x.happened", payload={}, search_space_id=1)
+
+    assert event.event_id
+    assert isinstance(event.occurred_at, datetime)
+
+
+def test_event_ids_are_unique_per_instance() -> None:
+    """Two events published with identical content are still distinct facts."""
+    first = Event(event_type="x.happened", payload={}, search_space_id=1)
+    second = Event(event_type="x.happened", payload={}, search_space_id=1)
+
+    assert first.event_id != second.event_id
+
+
+def test_event_survives_json_round_trip() -> None:
+    """Serialize → deserialize reproduces the event (subscribers queue it as JSON)."""
+    original = Event(
+        event_type="podcast.generated",
+        payload={"podcast_id": 9, "duration_s": 123.5},
+        search_space_id=3,
+    )
+
+    restored = Event.model_validate_json(original.model_dump_json())
+
+    assert restored == original