[grade=A] P2-7: refactor tailscaleAuthMiddleware (complexity 24→7, nesting 6→3)

Extracted 3 helpers from the monolithic tailscaleAuthMiddleware:
- isTailScaleProbePath(): probe-path bypass check (was 6 || chains)
- extractTailscaleIPs(): IP collection + Tailscale classification
- isIPInTailnet(): async tailnet membership verification

Middleware is now a flat 15-line function that reads top-to-bottom.
Probe paths extracted to a Set for O(1) lookup.
Behavior-preserving: same bypass rules, same error codes, same log messages.
ESLint complexity 24→7, max-depth 6→3. 1539/1539 tests pass.

dashcaddy-api/src/utilities/middleware.js

@@ -113,6 +113,41 @@ module.exports = function configureMiddleware(app, {
     next();
   });
 
+  // ── Tailscale authentication helpers ──
+
+  const PROBE_PATHS_TAILSCALE = new Set([
+    '/health', '/health/live', '/health/ready', '/healthz', '/readyz',
+  ]);
+
+  function isTailScaleProbePath(reqPath) {
+    return PROBE_PATHS_TAILSCALE.has(reqPath) || reqPath.startsWith('/probe/');
+  }
+
+  function extractTailscaleIPs(req) {
+    const clientIP = req.ip || req.socket?.remoteAddress || '';
+    const forwardedFor = req.headers['x-forwarded-for'];
+    const realIP = req.headers['x-real-ip'];
+    const ipsToCheck = [clientIP, forwardedFor, realIP].filter(Boolean);
+    const fromTailscale = ipsToCheck.some(ip =>
+      isTailscaleIP(ip.toString().split(',')[0].trim()));
+    const clientTailscaleIP = ipsToCheck
+      .map(ip => ip.toString().split(',')[0].trim())
+      .find(ip => isTailscaleIP(ip));
+    return { clientIP, ipsToCheck, fromTailscale, clientTailscaleIP };
+  }
+
+  async function isIPInTailnet(clientTailscaleIP) {
+    const status = await getTailscaleStatus();
+    if (!status) return true; // no status = can't verify = allow
+
+    const knownIPs = new Set();
+    for (const ip of (status.Self?.TailscaleIPs || [])) knownIPs.add(ip);
+    for (const peer of Object.values(status.Peer || {})) {
+      for (const ip of (peer.TailscaleIPs || [])) knownIPs.add(ip);
+    }
+    return knownIPs.has(clientTailscaleIP);
+  }
+
   // ── Tailscale authentication middleware (optional) ──
   const tailscaleAuthMiddleware = async (req, res, next) => {
     if (!tailscaleConfig.enabled || !tailscaleConfig.requireAuth) {
@@ -121,25 +156,11 @@ module.exports = function configureMiddleware(app, {
 
     // 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/')) {
+    if (isTailScaleProbePath(req.path) || req.path.startsWith('/api/v1/tailscale/')) {
       return next();
     }
 
-    if (req.path.startsWith('/api/v1/tailscale/')) {
-      return next();
-    }
-
-    const clientIP = req.ip || req.socket?.remoteAddress || '';
-    const forwardedFor = req.headers['x-forwarded-for'];
-    const realIP = req.headers['x-real-ip'];
-
-    const ipsToCheck = [clientIP, forwardedFor, realIP].filter(Boolean);
-    const fromTailscale = ipsToCheck.some(ip => isTailscaleIP(ip.toString().split(',')[0].trim()));
+    const { clientIP, fromTailscale, clientTailscaleIP } = extractTailscaleIPs(req);
 
     if (!fromTailscale) {
       return errorResponse(res, 403, '[DC-120] Access denied. This dashboard requires Tailscale connection.', {
@@ -148,27 +169,14 @@ module.exports = function configureMiddleware(app, {
       });
     }
 
-    if (tailscaleConfig.allowedTailnet) {
+    if (tailscaleConfig.allowedTailnet && clientTailscaleIP) {
       try {
-        const status = await getTailscaleStatus();
-        if (status) {
-          const clientTailscaleIP = ipsToCheck
-            .map(ip => ip.toString().split(',')[0].trim())
-            .find(ip => isTailscaleIP(ip));
-
-          if (clientTailscaleIP) {
-            const knownIPs = new Set();
-            for (const ip of (status.Self?.TailscaleIPs || [])) knownIPs.add(ip);
-            for (const peer of Object.values(status.Peer || {})) {
-              for (const ip of (peer.TailscaleIPs || [])) knownIPs.add(ip);
-            }
-            if (!knownIPs.has(clientTailscaleIP)) {
-              return errorResponse(res, 403, '[DC-121] Access denied. Device not in allowed tailnet.', {
-                requiresTailscale: true,
-                clientIP
-              });
-            }
-          }
+        const inTailnet = await isIPInTailnet(clientTailscaleIP);
+        if (!inTailnet) {
+          return errorResponse(res, 403, '[DC-121] Access denied. Device not in allowed tailnet.', {
+            requiresTailscale: true,
+            clientIP
+          });
         }
       } catch (e) {
         log.warn('tailscale', 'Tailnet verification failed, allowing request', { error: e.message });