feat: mark generated and identity columns across writers and DBML

- bun: scanonly+generated for GENERATED columns; scanonly+identity for ALWAYS identity (PK gets identity only)
- gorm: <-:false+generated / identity, same primary key rule
- dbml: write GENERATED/IDENTITY column notes and read them back into the model
- pgsql: emit generation/identity clauses in migration create-table and add-column, no DEFAULT
- readmes updated

pkg/writers/pgsql/migration_writer.go

@@ -442,6 +442,7 @@ func (w *MigrationWriter) generateAlterTableScripts(schema *models.Schema, model
 		if !exists {
 			// Column doesn't exist, add it
 			_, defaultVal := formatColumnDefaultSQL(modelCol)
+			generated, identity := columnGenerationClauses(modelCol)
 
 			sql, err := w.executor.ExecuteAddColumn(AddColumnData{
 				SchemaName: schema.Name,
@@ -450,6 +451,8 @@ func (w *MigrationWriter) generateAlterTableScripts(schema *models.Schema, model
 				ColumnType: effectiveColumnSQLType(modelCol),
 				Default:    defaultVal,
 				NotNull:    modelCol.NotNull,
+				Generated:  generated,
+				Identity:   identity,
 			})
 			if err != nil {
 				return nil, err