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/core/models/dashboarddefault.py

@@ -0,0 +1,28 @@
+"""Dashboard default model: visitor-IP -> business-unit mapping.
+
+A shopfloor / lobby kiosk display resolves which business unit to show by the
+display PC's IP address. Powers the visitor-location lookup the shopfloor and
+TV dashboards call when no explicit business unit is given.
+"""
+
+from shopdb.extensions import db
+from .base import BaseModel
+
+
+class DashboardDefault(BaseModel):
+    """Maps a display PC IP address to the business unit it should show."""
+    __tablename__ = 'dashboarddefaults'
+
+    dashboarddefaultid = db.Column(db.Integer, primary_key=True)
+    ipaddress = db.Column(db.String(50), unique=True, nullable=False)
+    businessunitid = db.Column(
+        db.Integer,
+        db.ForeignKey('businessunits.businessunitid'),
+        nullable=False
+    )
+    description = db.Column(db.String(255))
+
+    businessunit = db.relationship('BusinessUnit')
+
+    def __repr__(self):
+        return f"<DashboardDefault {self.ipaddress} -> {self.businessunitid}>"