Fix plugin contract violation and installer defaults mismatch

Fix 1: Remove duplicate Setting import violating plugin contract
- plugins/endpoints/api/routes.py:3551 imported shopdb.core.models.Setting
- Setting already imported from shopdb.api at line 21
- Removed local import to comply with plugin contract (ADR-001)
- Test: test_plugins_only_import_contract_surface

Fix 2: Align installer defaults with measuringtools manifest
- measuringtools manifest declares default_enabled: true
- Installer (ShopDBFlask.iss) excluded it from PluginDefault()
- Removed measuringtools from exclusion list (now 4 excluded, not 5)
- Exclusions now match manifests: usb, employees, geenforce, printedparts
- Test: test_wizard_defaults_match_the_manifests

Historical note: measuringtools was promoted from default_enabled false to
true but the installer config was never updated to reflect this change.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

plugins/endpoints/api/routes.py

@@ -3548,7 +3548,6 @@ def _send_operator_report_alert(report, asset_id, hostname):
             current_app.logger.warning('Failed to generate screenshot URL: %s', e)
 
     # Get PC access domain to build FQDN
-    from shopdb.core.models import Setting
     pc_access_domain_setting = Setting.query.filter_by(key='pc_access_domain').first()
     pc_access_domain = pc_access_domain_setting.value if pc_access_domain_setting else ''