dashboarddefaults: key display mappings by stable FQDN (from BIOS serial), IP fallback

A display's DHCP IP can change; its FQDN (F<serial>.<domain>, domain from the
display_fqdn_domain setting) is stable and the collector already reports the
serial. Add a nullable unique fqdn column (varchar191 so the index fits utf8mb4
without innodb_large_prefix), make ipaddress nullable, and require fqdn OR ip.
visitor-location + display-role resolve by FQDN first, then IP; create/update
accept fqdn. Core migration 7d31, verified up/down/idempotent on MySQL 5.6.
'Business unit' wording -> 'location' in the validation messages.

migrations/versions/7d31_dashboarddefault_fqdn.py

@@ -0,0 +1,62 @@
+"""dashboarddefaults: add fqdn, make ipaddress nullable.
+
+Map a display PC to its role/location by its STABLE FQDN (derived from the BIOS
+serial, F<serial>.<domain>) instead of only its DHCP-changeable IP. IP stays as
+a nullable fallback. Idempotent.
+"""
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision = '7d31_dashboarddefault_fqdn'
+down_revision = '7d30_apitoken_resourcescopes'
+branch_labels = None
+depends_on = None
+
+
+def _has_column(bind, table, column):
+    inspector = sa.inspect(bind)
+    if table not in inspector.get_table_names():
+        return False
+    return column in {c['name'] for c in inspector.get_columns(table)}
+
+
+def _col_nullable(bind, table, column):
+    inspector = sa.inspect(bind)
+    for c in inspector.get_columns(table):
+        if c['name'] == column:
+            return c.get('nullable', True)
+    return True
+
+
+def _has_index(bind, table, index):
+    inspector = sa.inspect(bind)
+    if table not in inspector.get_table_names():
+        return False
+    return index in {i['name'] for i in inspector.get_indexes(table)}
+
+
+def upgrade():
+    bind = op.get_bind()
+    if not _has_column(bind, 'dashboarddefaults', 'fqdn'):
+        op.add_column('dashboarddefaults',
+                      sa.Column('fqdn', sa.String(191), nullable=True))
+    if not _has_index(bind, 'dashboarddefaults', 'idx_dashboarddefaults_fqdn'):
+        op.create_index('idx_dashboarddefaults_fqdn', 'dashboarddefaults',
+                        ['fqdn'], unique=True)
+    # IP is now optional (an FQDN-only mapping has no IP).
+    if not _col_nullable(bind, 'dashboarddefaults', 'ipaddress'):
+        op.alter_column('dashboarddefaults', 'ipaddress',
+                        existing_type=sa.String(50), nullable=True)
+
+
+def downgrade():
+    bind = op.get_bind()
+    if _has_index(bind, 'dashboarddefaults', 'idx_dashboarddefaults_fqdn'):
+        op.drop_index('idx_dashboarddefaults_fqdn',
+                      table_name='dashboarddefaults')
+    if _has_column(bind, 'dashboarddefaults', 'fqdn'):
+        op.drop_column('dashboarddefaults', 'fqdn')
+    # ipaddress is left nullable: reverting to NOT NULL could fail on FQDN-only
+    # rows that legitimately have no IP.