Add dashboard defaults (visitor-IP -> business-unit) for kiosk displays

Classic feature gap: a shopfloor/lobby kiosk auto-selects which business unit to
show based on the display PC's IP (classic dashboarddefaults table +
apivisitorlocation.asp). For the main admin dashboard this does nothing - it is
kiosk/visitor-display infra.

- Model: DashboardDefault (dashboarddefaults: ipaddress unique, businessunitid
  FK, description). Migration 7d01_dashboarddefaults (head).
- API (core, /api/dashboarddefaults): CRUD + GET /visitor-location that resolves
  the calling display's business unit from its IP (X-Forwarded-For/remote_addr,
  or explicit ?ipaddress=); unmapped IP returns a null businessunitid, not an
  error. Unauthenticated resolve (kiosks); writes are admin.
- Frontend: ShopfloorDashboard auto-selects its business unit via visitor-location
  on load when none is chosen; Settings > Dashboard Defaults CRUD page +
  dashboardDefaultsApi client.

Tests: create + resolve by IP -> BU; unmapped IP -> null; duplicate IP 409.
191 tests pass, naming green, app boots, endpoint + admin page verified live.

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

shopdb/__init__.py

@@ -101,6 +101,7 @@ CORE_BLUEPRINT_NAMES = (
     'locations',
     'operatingsystems',
     'dashboard',
+    'dashboarddefaults',
     'applications',
     'search',
     'reports',