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.

decnet/web/api.py

@@ -1,4 +1,8 @@
 import asyncio
+# Local binding for the DB-retry sleep so tests can patch it without
+# affecting `asyncio.sleep` globally (which would otherwise starve the
+# heartbeat / worker loops that share the interpreter's asyncio module).
+from asyncio import sleep as _retry_sleep
 import os
 import traceback
 import uuid
@@ -75,7 +79,7 @@ async def lifespan(app: FastAPI) -> AsyncGenerator[None, None]:
             log.warning("DB init attempt %d/5 failed: %s", attempt, exc)
             if attempt == 5:
                 log.error("DB failed to initialize after 5 attempts — startup may be degraded")
-            await asyncio.sleep(0.5)
+            await _retry_sleep(0.5)
 
     # Conditionally enable OpenTelemetry tracing
     from decnet.telemetry import setup_tracing