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/writer.go

@@ -462,6 +462,12 @@ func (w *Writer) GenerateAlterColumnTypeStatements(schema *models.Schema) ([]str
 	for _, table := range schema.Tables {
 		columns := getSortedColumns(table.Columns)
 		for _, col := range columns {
+			if col.Generated || col.Identity {
+				// Type is derived from the generation expression (generated columns) or
+				// tied to the backing sequence (identity columns); leave both alone here
+				// rather than risk an ALTER COLUMN TYPE Postgres won't accept cleanly.
+				continue
+			}
 			targetType := effectiveAlterColumnSQLType(col)
 			stmt, err := w.executor.ExecuteAlterColumnTypeWithCheck(AlterColumnTypeWithCheckData{
 				SchemaName:      schema.Name,
@@ -492,6 +498,14 @@ func (w *Writer) GenerateAlterColumnDefaultStatements(schema *models.Schema) ([]
 	for _, table := range schema.Tables {
 		columns := getSortedColumns(table.Columns)
 		for _, col := range columns {
+			if col.Generated || col.Identity {
+				// Generated columns: Postgres stores the generation expression in
+				// pg_attrdef just like a real default, so the guard template would see
+				// it as a stray default and emit DROP DEFAULT, which Postgres rejects.
+				// Identity columns never carry a DEFAULT at all (they're driven by a
+				// backing sequence), so there is nothing for this generator to manage.
+				continue
+			}
 			setDefault, defaultVal := formatColumnDefaultSQL(col)
 			stmt, err := w.executor.ExecuteAlterColumnDefaultWithCheck(AlterColumnDefaultWithCheckData{
 				SchemaName:   schema.Name,
@@ -605,13 +619,25 @@ func (w *Writer) generateColumnDefinition(col *models.Column) string {
 
 	parts = append(parts, effectiveColumnSQLType(col))
 
+	// GENERATED ALWAYS ... STORED columns are computed from an expression and
+	// cannot also carry a DEFAULT, so this takes the place of the DEFAULT clause.
+	if col.Generated && col.GenerationExpression != "" {
+		parts = append(parts, fmt.Sprintf("GENERATED ALWAYS AS (%s) STORED", col.GenerationExpression))
+	}
+
+	// GENERATED { ALWAYS | BY DEFAULT } AS IDENTITY columns are driven by an
+	// internal sequence, not a literal DEFAULT, and cannot carry one either.
+	if col.Identity && col.IdentityGeneration != "" {
+		parts = append(parts, fmt.Sprintf("GENERATED %s AS IDENTITY", col.IdentityGeneration))
+	}
+
 	// NOT NULL
 	if col.NotNull {
 		parts = append(parts, "NOT NULL")
 	}
 
 	// DEFAULT
-	if col.Default != nil {
+	if !col.Generated && !col.Identity && col.Default != nil {
 		switch v := col.Default.(type) {
 		case string:
 			parts = append(parts, fmt.Sprintf("DEFAULT %s", writers.QuoteDefaultValue(stripBackticks(v), col.Type)))