Add gauge lab + maintenance asset references with enable/disable toggles

Dedicated assets.gaugelabreference and assets.maintenancereference columns
(distinct from assetnumber), surfaced on equipment. Add global per-identifier
enable/disable settings (gauge/maintenance/FQDN) read via a shared composable
and toggled in System Settings.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/composables/identifierSettings.js

@@ -0,0 +1,36 @@
+// Global enable/disable flags for optional asset identifiers, read from the
+// settings table. Loaded once and shared across components. Defaults to
+// enabled when a flag is missing so a fresh install shows the identifiers.
+import { reactive } from 'vue'
+import { settingsApi } from '../api'
+
+const identifierflags = reactive({
+  gaugelabreference: true,
+  maintenancereference: true,
+  fqdn: true,
+  loaded: false
+})
+
+let inflight = null
+
+export function useIdentifierFlags() {
+  if (!identifierflags.loaded && !inflight) {
+    inflight = settingsApi.list()
+      .then(({ data }) => {
+        const map = {}
+        ;(data.data || []).forEach(s => { map[s.key] = s.value })
+        if ('identifier_gaugelabreference_enabled' in map) {
+          identifierflags.gaugelabreference = map.identifier_gaugelabreference_enabled !== false
+        }
+        if ('identifier_maintenancereference_enabled' in map) {
+          identifierflags.maintenancereference = map.identifier_maintenancereference_enabled !== false
+        }
+        if ('identifier_fqdn_enabled' in map) {
+          identifierflags.fqdn = map.identifier_fqdn_enabled !== false
+        }
+        identifierflags.loaded = true
+      })
+      .catch(() => { identifierflags.loaded = true })
+  }
+  return identifierflags
+}