fix(web/api): scope DB-retry sleep so tests don't starve background tasks

test_lifespan_db_retry patched decnet.web.api.asyncio.sleep to skip the
DB-retry backoff. Problem: asyncio is a shared module — the patch leaks
to every caller that looked up asyncio.sleep via `import asyncio`,
including run_health_heartbeat's own sleep loop. That heartbeat task
spawns inside the same lifespan; with its sleep mocked, the while-loop
spins tight, starves cancellation, and leaves an orphan task that
pytest-timeout eventually signals — surfacing as the 'Task exception
was never retrieved' warnings the user saw when running the suite.

Fix: give decnet.web.api a local binding `_retry_sleep = asyncio.sleep`
for the DB-retry wait, and have the test patch that instead. Narrowly
scoped, no impact on asyncio.sleep callers elsewhere.

Test timing before: 12s with --timeout=10 (interrupted by signal).
Test timing after: 0.58s. Full tests/web slice: 27s → 7.1s with the
spurious warnings gone.

tests/web/test_web_api.py

@@ -148,7 +148,10 @@ class TestLifespan:
         mock_repo.initialize = _failing_init
 
         with patch("decnet.web.api.repo", mock_repo):
-            with patch("decnet.web.api.asyncio.sleep", new_callable=AsyncMock):
+            # Patch only the local _retry_sleep binding — patching
+            # `asyncio.sleep` globally would starve the heartbeat loop's
+            # own sleep and leak the task past the test's lifetime.
+            with patch("decnet.web.api._retry_sleep", new_callable=AsyncMock):
                 with patch("decnet.web.api.log_ingestion_worker", return_value=asyncio.sleep(0)):
                     with patch("decnet.web.api.log_collector_worker", return_value=asyncio.sleep(0)):
                         with patch("decnet.web.api.attacker_profile_worker", return_value=asyncio.sleep(0)):