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/app.js

@@ -654,10 +654,10 @@ async function createApp() {
     logError: ctx.logError,
   }));
 
-  // Inline API routes
-  apiRouter.get('/health', (req, res) => {
-    ok(res, { status: 'ok', timestamp: new Date().toISOString() });
-  });
+  // Inline API routes (mounted under /api/v1 below)
+  // Note: /health lives at root only — see root-level health check below.
+  // Probes (/healthz, /readyz, /health/live, /health/ready) also at root only.
+  // Do NOT add another /api/v1/health route — it's been consolidated.
   
   apiRouter.get('/csrf-token', (req, res) => {
     ok(res, { token: req.csrfToken, headerName: CSRF_HEADER_NAME });
@@ -670,24 +670,33 @@ async function createApp() {
   // Mount at /api/v1 (canonical, single version)
   app.use('/api/v1', apiRouter);
 
-  // Root-level health check
-  app.get('/health', (req, res) => {
-    ok(res, { status: 'ok', timestamp: new Date().toISOString() });
-  });
+  // ===========================================================================
+  // Health probes — root-level, no auth, no CSRF, no rate limit.
+  //
+  // Two semantics, four paths:
+  //
+  //   LIVENESS  — "is the Node.js process alive?"
+  //     /health/live    (explicit, recommended)
+  //     /healthz        (k8s/Docker-standard alias)
+  //   READINESS — "are critical dependencies reachable?"
+  //     /health/ready   (explicit, recommended)
+  //     /readyz         (k8s/Docker-standard alias)
+  //
+  // k8s/Docker/Caddy call these to decide whether to RESTART or ROUTE TRAFFIC.
+  // They MUST stay cheap (no DB queries, no logging side effects, no auth).
+  //
+  // Plain /health is kept for backwards compatibility and returns the same
+  // payload as /health/live. Use /health/live or /healthz in new code.
+  // ===========================================================================
 
-  // Liveness probe — "is the process alive?"
-  // Always returns 200 unless the Node.js event loop is completely blocked.
-  // Used by k8s/Docker to decide whether to RESTART the container.
-  // DO NOT add dependency checks here — those belong in /health/ready.
-  app.get('/health/live', (req, res) => {
+  // Liveness — pure process check, no deps.
+  const livenessHandler = (req, res) => {
     ok(res, { status: 'alive', uptime: process.uptime() });
-  });
+  };
 
-  // Readiness probe — "is the app ready to serve traffic?"
-  // Checks critical dependencies: Docker daemon, Caddy admin API, config file.
-  // Returns 200 with details if all OK, 503 with failed components otherwise.
-  // Used by k8s/Docker to decide whether to ROUTE TRAFFIC to this instance.
-  app.get('/health/ready', boundAsyncHandler(async (req, res) => {
+  // Readiness — checks critical dependencies (config, services file,
+  // Docker daemon, Caddy admin). 200 if all OK, 503 if any failed.
+  const readinessHandler = boundAsyncHandler(async (req, res) => {
     const checks = {};
     let allOk = true;
 
@@ -753,7 +762,16 @@ async function createApp() {
       checks
     };
     ok(res, body, allOk ? 200 : 503);
-  }));
+  });
+
+  // Liveness paths
+  app.get('/health', livenessHandler);
+  app.get('/health/live', livenessHandler);
+  app.get('/healthz', livenessHandler);
+
+  // Readiness paths
+  app.get('/health/ready', readinessHandler);
+  app.get('/readyz', readinessHandler);
 
   // Lightweight probe endpoint
   app.get('/probe/:id', boundAsyncHandler(async (req, res) => {