sync-v2: queue-isolation routing test (acceptance §10.5)

Assert bulk backfills route to the bulk queue and interactive user actions to
the interactive queue, and that enqueue_job stamps _queue_name accordingly — the
mechanism that keeps a backfill burst from starving sends/flag changes.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/tests/test_queue_routing.py

@@ -0,0 +1,53 @@
+"""Interactive/bulk queue isolation (refactor §1, acceptance §10.5).
+
+Bulk backfills and interactive user actions land on separate arq queues (run by
+separate consumers with their own concurrency), so a burst of backfills can never
+starve sends/flag changes. Here we assert the routing that guarantees it.
+"""
+
+from __future__ import annotations
+
+from app import queue as app_queue
+from app.queue import QUEUE_BULK, QUEUE_INTERACTIVE, queue_for
+
+_INTERACTIVE_JOBS = [
+    "push_flags_job",
+    "move_job",
+    "move_thread_to_folder_job",
+    "delete_thread_job",
+    "send_job",
+    "push_draft_to_imap_job",
+    "remove_imap_draft_job",
+    "refresh_account_job",
+]
+
+
+def test_bulk_backfill_routes_to_bulk_queue() -> None:
+    assert queue_for("sync_bootstrap_account") == QUEUE_BULK
+
+
+def test_interactive_jobs_route_to_interactive_queue() -> None:
+    for name in _INTERACTIVE_JOBS:
+        assert queue_for(name) == QUEUE_INTERACTIVE
+
+
+async def test_enqueue_job_sets_queue_name(monkeypatch) -> None:  # type: ignore[no-untyped-def]
+    captured: list[tuple[str, str | None]] = []
+
+    class FakePool:
+        async def enqueue_job(self, name: str, *args: object, **kwargs: object) -> None:
+            captured.append((name, kwargs.get("_queue_name")))  # type: ignore[arg-type]
+            return None
+
+    async def fake_pool() -> FakePool:
+        return FakePool()
+
+    monkeypatch.setattr(app_queue, "get_arq_pool", fake_pool)
+
+    await app_queue.enqueue_job("sync_bootstrap_account", "a")
+    await app_queue.enqueue_job("send_job", "b")
+
+    assert captured == [
+        ("sync_bootstrap_account", QUEUE_BULK),
+        ("send_job", QUEUE_INTERACTIVE),
+    ]