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>

pkg/orm/model.go

@@ -162,6 +162,10 @@ func (m *Model) addMagicFields() {
 // AddField adds a field to this model.
 func (m *Model) AddField(f *Field) *Model {
 	f.model = m
+	// Skip duplicate field (ExtendModel may add same field from multiple modules)
+	if _, exists := m.fields[f.Name]; exists {
+		return m
+	}
 	m.fields[f.Name] = f
 	m.allFields[f.Name] = f
 	m.fieldOrder = append(m.fieldOrder, f.Name)