feat(models): add support for generated and identity columns

* extend Column struct with Generated, GenerationExpression, Identity, and IdentityGeneration fields
* update queryColumns to handle new column properties
* modify migration writer to avoid altering generated and identity columns
* add tests to ensure correct handling of generated and identity columns

pkg/writers/pgsql/migration_writer.go

@@ -433,6 +433,16 @@ func (w *MigrationWriter) generateAlterTableScripts(schema *models.Schema, model
 				Body:       sql,
 			}
 			scripts = append(scripts, script)
+		} else if currentCol.Generated || currentCol.Identity {
+			// GENERATED ALWAYS ... STORED columns and GENERATED ... AS IDENTITY
+			// columns are both driven by Postgres-side machinery (an expression, or
+			// a backing sequence + attidentity flag) that DBML cannot express, so
+			// the model side always looks like drift (missing type modifiers, no
+			// default, NOT NULL mismatches, etc). None of their properties are
+			// alterable the normal way (e.g. dropping a generated column's default
+			// requires DROP EXPRESSION, not DROP DEFAULT; identity columns have no
+			// default to alter at all), so these DB-side-only columns are left
+			// untouched rather than diffed.
 		} else if !columnsEqual(modelCol, currentCol) {
 			// Column exists but properties changed
 			if !columnTypesEqual(modelCol, currentCol) {