fix(monitoring): flatten CPU/mem data, add health summary, public + rate-limit monitoring/stats

Three coordinated fixes for the System Overview widget:

1. routes/monitoring.js — flatten getAllStats() shape from
   {current:{cpu:{percent},memory:{percent}}} to {cpu,memory,memoryUsage}
   so the widget's Number() coercion actually produces numbers, not NaN.
   Skill reference: references/totp-and-system-overview-pitfalls.md §3.

2. routes/health.js — add summary block to /health-checks/status response.
   Widget looks for {healthy, unhealthy, total} but only per-service objects
   existed. Permissive on healthy side (up|healthy|online), strict on
   unhealthy (down|unhealthy|offline|error); anything else counted as
   unknown. Same skill §3 reference.

3. middleware.js — add /api/v1/monitoring/stats to PUBLIC_ROUTES and the
   rate-limit skip list. The widget polls it every 5s from the dashboard;
   cookie-auth works but listing it explicitly makes it future-proof
   against auth-cookie expiry and prevents per-second 429s.

End-to-end test (unauthenticated):
  GET /api/v1/monitoring/stats  -> {cpu: 8.71, memory: 0.37, ...}
  GET /api/v1/health-checks/status -> {summary: {healthy:11, unhealthy:4, total:15}}

dashcaddy-api/VERSION

@@ -1 +1 @@
-db14233
+a372d62