fix: make users.phone nullable to allow admin creation without phone

Phone is optional — admin users created via script don't have one.
Added migration 0010 to alter the column, updated create_admin.py to
pass None instead of empty string.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web/migrations/versions/0010_users_phone_nullable.py

@@ -0,0 +1,17 @@
+"""Make users.phone nullable."""
+revision = "0010"
+down_revision = "0009"
+branch_labels = None
+depends_on = None
+
+from alembic import op
+import sqlalchemy as sa
+
+
+def upgrade():
+    op.alter_column("users", "phone", existing_type=sa.String(20), nullable=True)
+
+
+def downgrade():
+    op.execute("UPDATE users SET phone = '' WHERE phone IS NULL")
+    op.alter_column("users", "phone", existing_type=sa.String(20), nullable=False)