feat: add middle_name field and map all three Evotor name fields

- Add middle_name column to users table (migration 0012)
- Map Evotor's second_name → last_name, middle_name → middle_name
  in /user/create webhook handler
- Update name fields on existing users when Evotor sends them

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

web/migrations/versions/0012_users_middle_name.py

@@ -0,0 +1,16 @@
+"""Add middle_name to users."""
+revision = "0012"
+down_revision = "0011"
+branch_labels = None
+depends_on = None
+
+from alembic import op
+import sqlalchemy as sa
+
+
+def upgrade():
+    op.add_column("users", sa.Column("middle_name", sa.String(100), nullable=True))
+
+
+def downgrade():
+    op.drop_column("users", "middle_name")