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/machines/MachineForm.vue

@@ -34,8 +34,8 @@
           </div>
         </div>
 
-        <div class="form-row" v-if="identifierflags.gaugelabreference || identifierflags.maintenancereference">
-          <div class="form-group" v-if="identifierflags.gaugelabreference">
+        <div class="form-row" v-if="isEnabled('gaugelabreference', 'equipment') || isEnabled('maintenancereference', 'equipment')">
+          <div class="form-group" v-if="isEnabled('gaugelabreference', 'equipment')">
             <label for="gaugelabreference">Gauge Lab Reference</label>
             <input
               id="gaugelabreference"
@@ -46,7 +46,7 @@
             <small class="form-help">Authoritative gauge lab asset reference (if tracked)</small>
           </div>
 
-          <div class="form-group" v-if="identifierflags.maintenancereference">
+          <div class="form-group" v-if="isEnabled('maintenancereference', 'equipment')">
             <label for="maintenancereference">Maintenance Reference</label>
             <input
               id="maintenancereference"
@@ -358,7 +358,7 @@ import Modal from '../../components/Modal.vue'
 import { currentTheme } from '../../stores/theme'
 import { useIdentifierFlags } from '../../composables/identifierSettings'
 
-const identifierflags = useIdentifierFlags()
+const { isEnabled } = useIdentifierFlags()
 
 const route = useRoute()
 const router = useRouter()