Consolidate settings into 8 hubs and fix security vulnerabilities

Settings Consolidation (37+ pages to 8 hubs):
- Asset Type Hub: Unified type management with plugin descriptors
- Model Catalog Hub: Vendors, types, models, operating systems
- Asset Configuration Hub: Statuses, relationships, fields, identifiers
- Security Hub: Authentication, tokens, users, audit
- Site Configuration Hub: Site settings, floor map, branding, imports
- Integrations Hub: ServiceNow, Zabbix, Dell warranty
- Locations Hub: Locations, business units, employees
- Network Infrastructure Hub: VLANs, subnets

Plugin Descriptor Pattern:
- Add get_asset_type_descriptor hook to BasePlugin
- Implement descriptors in 6 plugins (printers, computers, machines, network, measuringtools, locations)
- Add /api/pluginui/asset-type-descriptors and core-type-descriptors endpoints
- Generic SubtypesTab component renders CRUD UI from descriptor schema

Security Fixes:
- Fix stored XSS via javascript: URIs in Models and Support Teams
- Add URL scheme validation (allow only http/https) in create/update endpoints
- Fix plugin contract violations in endpoints plugin (use shopdb.api imports)
- Add missing requiresAdmin guard to printer model supplies route
- Add user-facing error messages to API token load failures

Integration Improvements:
- Add persistent tab navigation to ServiceNow, Zabbix, and Dell Warranty settings
- Standardize integration settings UI across all three pages
- Fix tab visibility when navigating between integrations

Tested with 94-agent orchestrated review for ADR compliance and security.

plugins/printers/plugin.py

@@ -678,3 +678,16 @@ class PrintersPlugin(BasePlugin):
             return None
         # _computer_by_hostname returns the (Computer, Asset) pair.
         return row[1].assetid
+
+    def get_asset_type_descriptor(self):
+        """Return descriptor for printer types management in Asset Type Hub."""
+        return {
+            'assettype': 'printer',
+            'subtype_endpoint': '/printers/types',
+            'subtype_id_field': 'printertypeid',
+            'subtype_label_field': 'printertype',
+            'subtype_description_field': 'description',
+            'subtype_color_field': 'color',
+            'optional_tabs': [],
+            'edit_permission': 'printers.manage',
+        }