Fix DB setup: savepoints for seed, deduplicate AddField, field fixes

Critical fixes for fresh DB creation:
- AddField now skips duplicates (ExtendModel from multiple modules
  was adding same field twice → duplicate column error)
- SeedWithSetup wrapped in savepoints per seed block (one failing
  INSERT no longer aborts entire transaction)
- sale.order.cancel: display_name → cancel_reason (avoid magic field clash)
- purchase: removed duplicate supplier_rank (already on res.partner)
- safeExec helper: SAVEPOINT + ROLLBACK TO on error

Fresh DB creation now works:
- /web/database/create → creates all tables, seeds data, returns session
- Login works immediately after creation
- All 191 models, 51 menus, 34 actions seeded

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

addons/sale/models/sale_report.go

@@ -290,7 +290,7 @@ func initSaleOrderWarnMsg() {
 
 	m.AddFields(
 		orm.Many2one("order_id", "sale.order", orm.FieldOpts{String: "Sale Order"}),
-		orm.Text("display_name", orm.FieldOpts{String: "Warning"}),
+		orm.Text("cancel_reason", orm.FieldOpts{String: "Cancellation Reason"}),
 	)
 
 	// action_cancel: Confirm the cancellation of the sale order.