DC-012: Add Kubernetes-style /healthz + /readyz probe aliases

Fresh users copy-pasting healthcheck blocks from k8s/Docker docs need
the standard short aliases. Without /healthz and /readyz they get
connection refused. This commit:

1. Adds /healthz + /readyz as root-level aliases for /health/live +
   /health/ready in src/app.js. Handler bodies DRYed into named
   functions (livenessHandler, readinessHandler) so a probe semantics
   change updates all five paths at once.

2. Removes the dead /api/v1/health*, /api/v1/health/live, /api/v1/health/ready
   registrations from PUBLIC_ROUTES and CSRF exclusion list — those
   routes were never actually mounted on the apiRouter (only root
   paths existed). Anyone probing /api/v1/health now gets a clean 404
   instead of being routed through to a duplicate root handler.

3. Adds bypass for the 5 probe paths in three places where it matters:
   - PUBLIC_ROUTES (no auth)
   - csrf-protection.js excludedPaths (no CSRF check)
   - middleware.js request-logging exclusion (k8s polling every 10s
     doesn't flood the audit log)
   - middleware.js Tailscale auth bypass (probes don't carry Tailscale
     identity headers)

4. Adds __tests__/health-probe-aliases.test.js (19 tests):
   - Alias equivalence (/healthz == /health/live, /readyz == /health/ready)
   - Back-compat (/health == /health/live)
   - Path consolidation (all 3 /api/v1/health* return 404)
   - Source-of-truth PUBLIC_ROUTES allowlist sync check
   - Source-of-truth src/app.js mount list sync check (catches drift
     between handler mount and middleware allowlist)

5. Documents probes in README (copy-paste docker-compose.yml +
   Kubernetes blocks) and user-guide (Health Probes section + System
   API table updated).

Post-fix: 941/941 tests pass (+19 new). Zero new ESLint warnings
introduced. The pre-existing warnings/errors in src/app.js line 906
('os' is not defined) and the empty blocks in logging.test.js are
not regressions from this commit.

dashcaddy-api/src/security/csrf-protection.js

@@ -136,12 +136,19 @@ function csrfValidationMiddleware(req, res, next) {
   }
 
   // Excluded paths that don't require CSRF validation
+  // Note: probe endpoints (/health, /health/live, /health/ready, /healthz,
+  // /readyz) are GET-only so they're already excluded by the safe-methods
+  // check above. Listed here for explicit safety in case any of them ever
+  // accept a POST in the future.
   const excludedPaths = [
     '/api/v1/totp/verify',
     '/api/v1/totp/verify-setup',
     '/api/v1/totp/setup',
     '/health',
-    '/api/v1/health',
+    '/health/live',
+    '/health/ready',
+    '/healthz',
+    '/readyz',
     // Machine-to-machine: publishing host POSTs here with its own shared-secret
     // header (X-DashCaddy-Notify-Secret) — browsers never reach this endpoint.
     '/api/v1/system/update-notify'