feat(ingester): publish system.log per committed batch (DEBT-031 worker 6)

Ingester connects the bus at startup, emits a batch-committed summary
(component/flushed/position) after each successful _flush_batch.  Zero-
row flushes are suppressed so the topic stays meaningful.

Complements the collector's per-line system.log publishes: collector
signals ingress, ingester signals DB-persisted progress.  Federation
forwarder (worker 8) will subscribe to the batch-committed leaf to
trigger its upstream push.

Bus stays optional: publish_safely swallows failures, get_bus() can
return None, DECNET_BUS_ENABLED=false leaves the ingestion loop fully
functional.

tests/web/test_ingester_bus.py

@@ -0,0 +1,83 @@
+"""Bus wiring for the ingester (DEBT-031, worker 6).
+
+The ingester emits one ``system.log`` event per DB-committed batch via
+``_publish_batch``.  Per-line noise lives on the collector side; the
+ingester's job is to signal "N rows landed in the DB up to offset P" so
+heartbeat / federation consumers can tail DB progress without polling
+the state table.
+"""
+from __future__ import annotations
+
+import asyncio
+
+import pytest
+import pytest_asyncio
+
+from decnet.bus.fake import FakeBus
+from decnet.web.ingester import _publish_batch
+
+
+@pytest_asyncio.fixture
+async def bus() -> FakeBus:
+    b = FakeBus()
+    await b.connect()
+    yield b
+    await b.close()
+
+
+@pytest.mark.asyncio
+async def test_publish_batch_fires_on_nonempty_flush(bus: FakeBus) -> None:
+    sub = bus.subscribe("system.log")
+    async with sub:
+        await _publish_batch(bus, flushed=17, position=4096)
+        event = await asyncio.wait_for(sub.__anext__(), timeout=2.0)
+
+    assert event.topic == "system.log"
+    assert event.type == "batch_committed"
+    assert event.payload == {
+        "component": "ingester",
+        "flushed": 17,
+        "position": 4096,
+    }
+
+
+@pytest.mark.asyncio
+async def test_publish_batch_skips_zero_row_flush(bus: FakeBus) -> None:
+    # An empty batch shouldn't pollute the topic — nothing to signal.
+    sub = bus.subscribe("system.log")
+    async with sub:
+        await _publish_batch(bus, flushed=0, position=0)
+        # Expect nothing within a short window.  asyncio.wait_for raises
+        # TimeoutError when no event arrives.
+        with pytest.raises(asyncio.TimeoutError):
+            await asyncio.wait_for(sub.__anext__(), timeout=0.2)
+
+
+@pytest.mark.asyncio
+async def test_publish_batch_is_noop_when_bus_is_none() -> None:
+    # Bus-disabled path: ingester passes bus=None into _publish_batch.
+    # Must be a safe no-op; no exceptions, no hangs.
+    await _publish_batch(None, flushed=5, position=123)
+
+
+@pytest.mark.asyncio
+async def test_publish_batch_swallows_bus_failures(monkeypatch) -> None:
+    # A dead bus must never break the ingestion loop.
+    class _ExplodingBus:
+        async def publish(self, *_args, **_kwargs):
+            raise RuntimeError("transport exploded")
+
+    await _publish_batch(_ExplodingBus(), flushed=3, position=42)
+
+
+@pytest.mark.asyncio
+async def test_ingester_degrades_cleanly_when_bus_disabled(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    from decnet.bus.factory import get_bus
+
+    monkeypatch.setenv("DECNET_BUS_ENABLED", "false")
+    b = get_bus(client_name="ingester")
+    await b.connect()
+    await b.publish("system.log", {"component": "ingester"}, event_type="batch_committed")
+    await b.close()