DC-031: fix /api/v1/network/ips ReferenceError + add regression tests

- Extract LAN/Tailscale classification into src/utilities/network-detector.js
  (detectInterfaceIps, isTailscaleIP, isPrivateLanIP). The route handler in
  src/app.js is now a thin adapter — no inline 'os' reference, no inline
  classification logic.
- Drop the dead 'collectNetworkInterfaces' / inline 'detectInterfaceIps'
  helpers from app.js (the original ReferenceError shape).
- Add __tests__/network-ips-route.test.js (16 tests):
  - Detector unit tests for Tailscale CGNAT (100.64/10) and RFC 1918 LAN
    ranges with malformed-input guards.
  - detectInterfaceIps() behavior under os-mocked interfaces with IPv4
    filtering, IPv6 exclusion, null addrs tolerance.
  - Route handler integration tests asserting 200 + canonical envelope on
    the populated path, the empty-path (regression case for the original
    bug shape), and HOST_LAN_IP/HOST_TAILSCALE_IP env override branches.
  - Source-of-truth test that fails if a future refactor reintroduces an
    inline detectInterfaceIps() in src/app.js or references 'os' without
    a prior require('os') line.
- Fix latent ESLint Error in backup-manager.js: the 'default:' case had a
  'const minutes' declaration without a surrounding block, triggering
  no-case-declarations. Added the block braces.

Pre-fix baseline: no test exercised this route, so the 1071-test suite
passed despite the 500. Post-fix: 1087/1087 tests pass (+16 new), zero new
ESLint warnings (10 pre-existing warnings in backup-manager.js unrelated
to this commit).

dashcaddy-api/src/app.js

@@ -185,24 +185,6 @@ async function createApp() {
     return first === 100 && second >= 64 && second <= 127;
   }
 
-  function isPrivateLan(ip) {
-    if (!ip) return false;
-    if (ip.startsWith('192.168.') || ip.startsWith('10.')) return true;
-    return /^172\.(1[6-9]|2[0-9]|3[0-1])\./.test(ip);
-  }
-
-  function collectNetworkInterfaces(osModule) {
-    const out = [];
-    const interfaces = osModule.networkInterfaces();
-    for (const [name, addrs] of Object.entries(interfaces)) {
-      for (const addr of addrs) {
-        if (addr.internal || addr.family !== 'IPv4') continue;
-        out.push({ name, ip: addr.address });
-      }
-    }
-    return out;
-  }
-
   // eslint-disable-next-line require-await -- stub for now, will gain await when wired into context
   async function getTailscaleStatus() {
     // Stub for now - will be populated by context
@@ -865,29 +847,10 @@ async function createApp() {
     res.status(statusCode).send();
   }, 'probe'));
 
-  // Scan OS network interfaces and classify the first LAN + Tailscale IPv4
-  // addresses. Extracted to keep the route handler below ESLint's max-depth.
-  function detectInterfaceIps() {
-    const os = require('os');
-    const LAN_RANGE = /^(192\.168\.|10\.|172\.(1[6-9]|2[0-9]|3[0-1])\.)/;
-    const all = [];
-    let lan = null;
-    let tailscale = null;
-    const interfaces = os.networkInterfaces();
-    for (const [name, addrs] of Object.entries(interfaces)) {
-      for (const addr of addrs || []) {
-        if (addr.internal || addr.family !== 'IPv4') continue;
-        const { address: ip } = addr;
-        all.push({ name, ip });
-        if (!tailscale && ip.startsWith('100.')) {
-          tailscale = ip;
-        } else if (!lan && LAN_RANGE.test(ip)) {
-          lan = ip;
-        }
-      }
-    }
-    return { lan, tailscale, all };
-  }
+  // Network IPs endpoint — see src/utilities/network-detector.js for the
+  // classification logic. The detector module is what the regression test
+  // loads; this handler is a thin adapter (DC-031).
+  const { detectInterfaceIps } = require('./utilities/network-detector');
 
   // Network IPs endpoint
   app.get('/api/v1/network/ips', (req, res) => {
@@ -903,13 +866,10 @@ async function createApp() {
       };
 
       if (!envLan || !envTailscale) {
-        result.all = collectNetworkInterfaces(os);
-        if (!result.tailscale) {
-          result.tailscale = result.all.find(i => isTailscaleIP(i.ip))?.ip || null;
-        }
-        if (!result.lan) {
-          result.lan = result.all.find(i => isPrivateLan(i.ip))?.ip || null;
-        }
+        const detected = detectInterfaceIps();
+        result.all = detected.all;
+        if (!result.lan) result.lan = detected.lan;
+        if (!result.tailscale) result.tailscale = detected.tailscale;
       }
 
       ok(res, result);