fix(app): cancel readiness loop on lifespan shutdown (review round 2)

_readiness_refresh_loop is started with tg.start_soon and loops forever with no
shutdown_event check. anyio waits for start_soon tasks on normal task-group exit
rather than cancelling them, so graceful shutdown hung until uvicorn's timeout.
Cancel the task group's scope after teardown() to stop the loop and any
stragglers, mirroring _maybe_login_flow_cleanup.

Also document the cache ttl_seconds startup-override and the inclusive is_stale
boundary.

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

nextcloud_mcp_server/observability/readiness.py

@@ -70,6 +70,7 @@ class ReadinessCache(BaseModel):
         if not self.statuses:
             return True
         current = time.monotonic() if now is None else now
+        # Inclusive boundary: exactly ttl_seconds old counts as stale.
         return any(
             current - status.checked_at >= self.ttl_seconds
             for status in self.statuses.values()