[IMP] website_sale_aplicoop: add sequence field to product.category for web ordering

- Add sequence field (default 10) to product.category with _order = "sequence, name"
- Inherit product.category tree view to add drag-handle widget
- Sort category hierarchy and available categories by sequence in controller
- Migration 18.0.1.9.0: add sequence column to product_category table
- Bump version to 18.0.1.9.0

website_sale_aplicoop/migrations/18.0.1.9.0/pre-migrate.py

@@ -0,0 +1,13 @@
+"""Add sequence field to product.category.
+
+Ensures the sequence column exists with a default of 10 for all existing rows.
+The ORM will also handle this on upgrade, but we add it explicitly so the
+column is present before any post-install logic runs.
+"""
+
+
+def migrate(cr, version):
+    cr.execute("""
+        ALTER TABLE product_category
+        ADD COLUMN IF NOT EXISTS sequence INTEGER NOT NULL DEFAULT 10;
+    """)