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_bus.py

@@ -0,0 +1,181 @@
+"""``EventBus`` contract: subscribe, publish (stamp + fan out), dispatch.
+
+Each test uses a fresh ``EventBus`` — no shared global state.
+"""
+
+from __future__ import annotations
+
+import pytest
+
+from app.event_bus import Event, EventBus
+
+pytestmark = pytest.mark.unit
+
+
+def _event() -> Event:
+    return Event(event_type="x.happened", payload={"k": "v"}, search_space_id=1)
+
+
+async def _noop(_event: Event) -> None:
+    return None
+
+
+async def _other(_event: Event) -> None:
+    return None
+
+
+# --- registry -------------------------------------------------------------
+
+
+def test_subscribe_then_subscribers_returns_the_handler() -> None:
+    bus = EventBus()
+    bus.subscribe(_noop)
+
+    assert _noop in bus.subscribers()
+
+
+def test_subscribe_is_idempotent_for_the_same_handler() -> None:
+    """Registering the same handler twice must not make it fire twice."""
+    bus = EventBus()
+    bus.subscribe(_noop)
+    bus.subscribe(_noop)
+
+    assert bus.subscribers().count(_noop) == 1
+
+
+def test_distinct_handlers_both_register() -> None:
+    bus = EventBus()
+    bus.subscribe(_noop)
+    bus.subscribe(_other)
+
+    registered = bus.subscribers()
+    assert _noop in registered
+    assert _other in registered
+
+
+def test_subscribers_returns_a_defensive_snapshot() -> None:
+    """Mutating the returned list must not corrupt the registry."""
+    bus = EventBus()
+    bus.subscribe(_noop)
+
+    snapshot = bus.subscribers()
+    snapshot.clear()
+
+    assert _noop in bus.subscribers()
+
+
+def test_subscribe_returns_handler_so_it_can_be_used_as_a_decorator() -> None:
+    bus = EventBus()
+    returned = bus.subscribe(_other)
+
+    assert returned is _other
+
+
+def test_two_buses_do_not_share_subscribers() -> None:
+    """The registry is per-instance, not global."""
+    a = EventBus()
+    b = EventBus()
+    a.subscribe(_noop)
+
+    assert _noop in a.subscribers()
+    assert _noop not in b.subscribers()
+
+
+# --- dispatch -------------------------------------------------------------
+
+
+async def test_dispatch_delivers_event_to_every_subscriber() -> None:
+    bus = EventBus()
+    seen: list[tuple[str, Event]] = []
+
+    async def first(event: Event) -> None:
+        seen.append(("first", event))
+
+    async def second(event: Event) -> None:
+        seen.append(("second", event))
+
+    bus.subscribe(first)
+    bus.subscribe(second)
+
+    event = _event()
+    await bus.dispatch(event)
+
+    assert ("first", event) in seen
+    assert ("second", event) in seen
+
+
+async def test_dispatch_isolates_a_failing_subscriber() -> None:
+    """A subscriber that raises must not stop a healthy one from running."""
+    bus = EventBus()
+    healthy_ran = False
+
+    async def boom(_event: Event) -> None:
+        raise RuntimeError("subscriber blew up")
+
+    async def healthy(_event: Event) -> None:
+        nonlocal healthy_ran
+        healthy_ran = True
+
+    bus.subscribe(boom)
+    bus.subscribe(healthy)
+
+    await bus.dispatch(_event())
+
+    assert healthy_ran is True
+
+
+async def test_dispatch_never_propagates_subscriber_errors() -> None:
+    """``dispatch`` itself must not raise even if every subscriber fails."""
+    bus = EventBus()
+
+    async def boom(_event: Event) -> None:
+        raise ValueError("nope")
+
+    bus.subscribe(boom)
+
+    await bus.dispatch(_event())  # must not raise
+
+
+async def test_dispatch_with_no_subscribers_is_a_noop() -> None:
+    bus = EventBus()
+    await bus.dispatch(_event())  # must not raise
+
+
+# --- publish --------------------------------------------------------------
+
+
+async def test_publish_builds_a_stamped_event_and_fans_it_out() -> None:
+    bus = EventBus()
+    received: list[Event] = []
+
+    async def handler(event: Event) -> None:
+        received.append(event)
+
+    bus.subscribe(handler)
+    await bus.publish("document.indexed", {"document_id": 42}, search_space_id=7)
+
+    assert len(received) == 1
+    event = received[0]
+    assert event.event_type == "document.indexed"
+    assert event.payload == {"document_id": 42}
+    assert event.search_space_id == 7
+    # Engine-stamped identity/time on the way through.
+    assert event.event_id
+    assert event.occurred_at
+
+
+async def test_publish_defaults_payload_to_empty_dict() -> None:
+    bus = EventBus()
+    received: list[Event] = []
+
+    async def handler(event: Event) -> None:
+        received.append(event)
+
+    bus.subscribe(handler)
+    await bus.publish("x.happened", search_space_id=1)
+
+    assert received[0].payload == {}
+
+
+async def test_publish_with_no_subscribers_is_a_noop() -> None:
+    await EventBus().publish("x.happened", search_space_id=1)  # must not raise