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>

shopdb/core/models/asset.py

@@ -74,6 +74,17 @@ class Asset(BaseModel, SoftDeleteMixin, AuditMixin):
         db.String(100),
         comment='Display name/alias'
     )
+    gaugelabreference = db.Column(
+        db.String(50),
+        index=True,
+        comment='Gauge lab asset reference (authoritative tag the gauge lab '
+                'assigns to equipment); distinct from assetnumber'
+    )
+    maintenancereference = db.Column(
+        db.String(50),
+        index=True,
+        comment='Maintenance system asset reference; distinct from assetnumber'
+    )
     serialnumber = db.Column(
         db.String(100),
         index=True,