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>

migrations/versions/7d01_dashboarddefaults.py

@@ -0,0 +1,37 @@
+"""Add dashboarddefaults (visitor-IP -> business-unit mapping)
+
+A shopfloor / lobby kiosk resolves which business unit to display from the
+display PC's IP. Powers the visitor-location lookup.
+
+Revision ID: 7d01_dashboarddefaults
+Revises: 7c04_fold_plugin_schema
+Create Date: 2026-06-27
+
+"""
+from alembic import op
+import sqlalchemy as sa
+
+
+revision = '7d01_dashboarddefaults'
+down_revision = '7c04_fold_plugin_schema'
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    op.create_table(
+        'dashboarddefaults',
+        sa.Column('dashboarddefaultid', sa.Integer(), primary_key=True),
+        sa.Column('ipaddress', sa.String(length=50), nullable=False, unique=True),
+        sa.Column('businessunitid', sa.Integer(), nullable=False),
+        sa.Column('description', sa.String(length=255), nullable=True),
+        sa.Column('createddate', sa.DateTime(), nullable=True),
+        sa.Column('modifieddate', sa.DateTime(), nullable=True),
+        sa.Column('isactive', sa.Boolean(), nullable=True),
+        sa.ForeignKeyConstraint(['businessunitid'], ['businessunits.businessunitid'],
+                                name='fk_dashboarddefaults_businessunit'),
+    )
+
+
+def downgrade():
+    op.drop_table('dashboarddefaults')