feat(ingester): graceful mid-walk cancel on Stop — B4 cancel (plan #708)

Owning the walk lets Stop interrupt a live backfill chunk instead of letting it
run to its ~14.5-min time-box. ingest_core.run now polls _backfill_state at each
page boundary (a short SELECT, never held across the walk) and bails with PARTIAL
when an operator Stop has popped it. Latched on the first observed "running"
state so a run invoked without one (unit test / stale call) never self-cancels.
Progress is already checkpointed per-page, so a restart resumes from the cursor;
Stop clears it for a clean reset. No UI change — the existing Stop button now
just takes effect immediately.

Tests: _still_running reads the state; a latched run bails PARTIAL at the next
boundary when the state disappears (only the pre-cancel post ran).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_patreon_ingester.py

@@ -382,6 +382,50 @@ async def test_backfill_budget_cut_returns_partial_with_progress(
     assert result.cursor == "CUR2"
 
 
+@pytest.mark.asyncio
+async def test_still_running_reads_backfill_state(source_id, sync_engine, tmp_path, db):
+    """plan #708 B4: _still_running reflects the source's `_backfill_state` — the
+    flag an operator Stop pops to cancel a live chunk."""
+    ing = _ingester(sync_engine, tmp_path, _FakeClient([]), _FakeDownloader(tmp_path))
+    assert ing._still_running(source_id) is False  # absent → not running
+    src = (await db.execute(select(Source).where(Source.id == source_id))).scalar_one()
+    src.config_overrides = {"_backfill_state": "running"}
+    await db.commit()
+    assert ing._still_running(source_id) is True
+
+
+@pytest.mark.asyncio
+async def test_backfill_stops_when_operator_cancels_mid_walk(
+    source_id, sync_engine, tmp_path,
+):
+    """plan #708 B4: with the running state latched, a later page boundary that
+    finds it gone (Stop) bails the chunk with PARTIAL instead of finishing."""
+    pages = [
+        ("CUR2", [("p1", [_media("p1", 1)])]),
+        ("CUR3", [("p2", [_media("p2", 1)])]),
+        ("CUR4", [("p3", [_media("p3", 1)])]),
+    ]
+    downloader = _FakeDownloader(tmp_path)
+    ing = _ingester(sync_engine, tmp_path, _FakeClient(pages), downloader)
+
+    # Latch on the page-2 boundary (running), then "Stop" before page 3.
+    calls = {"n": 0}
+
+    def fake_still_running(_source_id):
+        calls["n"] += 1
+        return calls["n"] == 1
+
+    ing._still_running = fake_still_running
+
+    result = ing.run(
+        source_id=source_id, campaign_id="c1", artist_slug="ingest",
+        url="https://patreon.com/ingest", mode="backfill",
+    )
+    assert result.error_type == ErrorType.PARTIAL
+    assert "Stopped by operator" in result.error_message
+    assert downloader.download_calls == 1  # only p1 ran; cancelled before p2
+
+
 # --- recovery -------------------------------------------------------------