Make gauge/maintenance identifiers per-asset-type and extend to all types

Optional asset identifiers (gauge lab reference, maintenance reference, FQDN)
were global per-identifier and only surfaced on equipment. Now they are
toggleable per asset type and rendered on every asset type.

- settings: replace 3 global identifier toggles with a per-type matrix. New
  keys identifier_<name>_<assettype>_enabled (3 identifiers x 4 types).
  IDENTIFIER_LABELS / IDENTIFIER_ASSETTYPES constants drive the seed (API seed
  and CLI seed settings).
- composable: identifierSettings now exposes isEnabled(name, assettype),
  per-type flag winning over the legacy global key, defaulting on.
- backend writes: computers, network, printers asset create + update now
  accept gaugelabreference and maintenancereference (equipment already did).
  Reads already flowed through Asset.to_dict.
- frontend: Settings page renders an identifier x asset-type toggle matrix.
  Equipment, PC, printer, network forms and detail pages show gauge/maintenance
  (and FQDN where applicable) gated by isEnabled(name, type).

Legacy global identifier_<name>_enabled keys are still honored as a fallback
for older installs. SystemSettings toggles upsert (create on 404) so a deploy
that has not re-seeded still works on first toggle.

144 tests pass, naming/style check green, frontend builds. Verified live:
matrix renders, PC form shows the fields, PUT persists gauge/maintenance on a
PC and reads back.

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

frontend/src/views/pcs/PCDetail.vue

@@ -16,7 +16,7 @@
         <div class="hero-content">
           <div class="hero-title">
             <h1>{{ computer.assetnumber }}</h1>
-            <span v-if="identifierflags.fqdn && computer.computer?.hostname" class="hero-alias">{{ computer.computer.hostname }}</span>
+            <span v-if="isEnabled('fqdn', 'computer') && computer.computer?.hostname" class="hero-alias">{{ computer.computer.hostname }}</span>
           </div>
           <div class="hero-meta">
             <span class="badge badge-lg badge-info">Computer</span>
@@ -57,7 +57,7 @@
                 <span class="info-label">Name</span>
                 <span class="info-value">{{ computer.name }}</span>
               </div>
-              <div class="info-row" v-if="identifierflags.fqdn && computer.computer?.hostname">
+              <div class="info-row" v-if="isEnabled('fqdn', 'computer') && computer.computer?.hostname">
                 <span class="info-label">Hostname</span>
                 <span class="info-value mono">{{ computer.computer.hostname }}</span>
               </div>
@@ -65,6 +65,14 @@
                 <span class="info-label">Serial Number</span>
                 <span class="info-value mono">{{ computer.serialnumber }}</span>
               </div>
+              <div class="info-row" v-if="isEnabled('gaugelabreference', 'computer') && computer.gaugelabreference">
+                <span class="info-label">Gauge Lab Reference</span>
+                <span class="info-value mono">{{ computer.gaugelabreference }}</span>
+              </div>
+              <div class="info-row" v-if="isEnabled('maintenancereference', 'computer') && computer.maintenancereference">
+                <span class="info-label">Maintenance Reference</span>
+                <span class="info-value mono">{{ computer.maintenancereference }}</span>
+              </div>
             </div>
           </div>
 
@@ -210,7 +218,7 @@ import LocationMapTooltip from '../../components/LocationMapTooltip.vue'
 import { useIdentifierFlags } from '../../composables/identifierSettings'
 
 const route = useRoute()
-const identifierflags = useIdentifierFlags()
+const { isEnabled } = useIdentifierFlags()
 
 const loading = ref(true)
 const computer = ref(null)