displays: single display type with IP-driven role (dashboard/lobby/kiosk)

One 'display' image resolves what it shows from its own IP, like the existing
visitor-location BU mapping. Extend DashboardDefault with displayrole
(dashboard|lobby|partskiosk; migration 7d28, businessunitid now nullable since
only the dashboard role needs one) + a role->path map. New unauthenticated
GET /api/dashboarddefaults/display-role returns {role, path, businessunitid}
for the caller IP. Settings UI gains a Display selector, showing the business
unit only for the dashboard role.

migrations/versions/7d28_dashboarddefault_displayrole.py

@@ -0,0 +1,63 @@
+"""Add dashboarddefaults.displayrole; make businessunitid nullable
+
+One "display" PC image resolves its role (dashboard / lobby / partskiosk) from
+its IP. Existing rows are dashboard mappings, so displayrole defaults to
+'dashboard'. Lobby / kiosk roles need no business unit, so businessunitid
+becomes nullable.
+
+Idempotent guards on column presence / nullability.
+
+Revision ID: 7d28_dashboarddefault_displayrole
+Revises: 7d27_roles_color
+Create Date: 2026-07-21
+
+"""
+from alembic import op
+import sqlalchemy as sa
+
+
+revision = '7d28_dashboarddefault_displayrole'
+down_revision = '7d27_roles_color'
+branch_labels = None
+depends_on = None
+
+
+def _cols(insp):
+    return {c['name']: c for c in insp.get_columns('dashboarddefaults')}
+
+
+def upgrade():
+    bind = op.get_bind()
+    insp = sa.inspect(bind)
+    if 'dashboarddefaults' not in insp.get_table_names():
+        return
+    cols = _cols(insp)
+
+    if 'displayrole' not in cols:
+        op.add_column('dashboarddefaults', sa.Column(
+            'displayrole', sa.String(length=20),
+            nullable=False, server_default='dashboard'))
+
+    if 'businessunitid' in cols and not cols['businessunitid']['nullable']:
+        op.alter_column('dashboarddefaults', 'businessunitid',
+                        existing_type=sa.Integer(), nullable=True)
+
+
+def downgrade():
+    bind = op.get_bind()
+    insp = sa.inspect(bind)
+    if 'dashboarddefaults' not in insp.get_table_names():
+        return
+    cols = _cols(insp)
+
+    # Only restore NOT NULL if no rows would violate it.
+    if 'businessunitid' in cols and cols['businessunitid']['nullable']:
+        nulls = bind.execute(sa.text(
+            'SELECT COUNT(*) FROM dashboarddefaults '
+            'WHERE businessunitid IS NULL')).scalar()
+        if not nulls:
+            op.alter_column('dashboarddefaults', 'businessunitid',
+                            existing_type=sa.Integer(), nullable=False)
+
+    if 'displayrole' in cols:
+        op.drop_column('dashboarddefaults', 'displayrole')