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/utilities/middleware.js

@@ -96,7 +96,14 @@ module.exports = function configureMiddleware(app, {
     res.on('finish', () => {
       const duration = Date.now() - start;
       metrics.recordRequest(req.method, req.path, res.statusCode, duration);
-      if (req.path !== '/health' && req.path !== '/api/v1/health') {
+      // Skip noisy per-request logging for probe endpoints — k8s/Docker
+      // hit these every few seconds and would flood the audit log.
+      const isProbe = req.path === '/health'
+        || req.path === '/health/live'
+        || req.path === '/health/ready'
+        || req.path === '/healthz'
+        || req.path === '/readyz';
+      if (!isProbe) {
         const level = res.statusCode >= 500 ? 'error' : res.statusCode >= 400 ? 'warn' : 'debug';
         log[level]('http', `${req.method} ${req.path} ${res.statusCode}`, {
           ms: duration, ip: req.ip, id: req.id
@@ -112,7 +119,14 @@ module.exports = function configureMiddleware(app, {
       return next();
     }
 
-    if (req.path === '/health' || req.path === '/api/v1/health' || req.path.startsWith('/probe/')) {
+    // Probe endpoints bypass Tailscale auth — k8s/Docker healthchecks
+    // don't carry a Tailscale identity header.
+    if (req.path === '/health'
+      || req.path === '/health/live'
+      || req.path === '/health/ready'
+      || req.path === '/healthz'
+      || req.path === '/readyz'
+      || req.path.startsWith('/probe/')) {
       return next();
     }
 
@@ -294,12 +308,14 @@ module.exports = function configureMiddleware(app, {
   })();
 
   const PUBLIC_ROUTES = [
+    // Health probes — root-level only. See src/app.js for the handler block.
+    // Both the explicit (/health/live, /health/ready) and k8s-standard
+    // (/healthz, /readyz) aliases are unauthenticated by design.
     { path: '/health',                   exact: true },
     { path: '/health/live',              exact: true },
     { path: '/health/ready',             exact: true },
-    { path: '/api/v1/health',            exact: true },
-    { path: '/api/v1/health/live',       exact: true },
-    { path: '/api/v1/health/ready',      exact: true },
+    { path: '/healthz',                  exact: true },
+    { path: '/readyz',                   exact: true },
     { path: '/probe/',                   prefix: true },
     { path: '/api/v1/tailscale/',        prefix: true },
     { path: '/api/v1/totp/config',        exact: true,  method: 'GET' },