feat(migration): add fallback for column type conversion failures

* implement renaming of old column and adding new column on conversion failure
* update templates and tests to support new behavior

pkg/writers/pgsql/migration_writer.go

@@ -442,12 +442,14 @@ func (w *MigrationWriter) generateAlterTableScripts(schema *models.Schema, model
 		} else if !columnsEqual(modelCol, currentCol) {
 			// Column exists but properties changed
 			if !columnTypesEqual(modelCol, currentCol) {
-				sql, err := w.executor.ExecuteAlterColumnType(AlterColumnTypeData{
-					SchemaName: schema.Name,
-					TableName:  modelTable.Name,
-					ColumnName: modelCol.Name,
-					NewType:    effectiveAlterColumnSQLType(modelCol),
-					UsingExpr:  buildAlterColumnUsingExpression(modelCol.Name, effectiveAlterColumnSQLType(modelCol)),
+				newType := effectiveAlterColumnSQLType(modelCol)
+				sql, err := w.executor.ExecuteAlterColumnTypeWithFallback(AlterColumnTypeWithFallbackData{
+					SchemaName:    schema.Name,
+					TableName:     modelTable.Name,
+					ColumnName:    modelCol.Name,
+					NewType:       newType,
+					UsingExpr:     buildAlterColumnUsingExpression(modelCol.Name, newType),
+					OldColumnName: renamedColumnName(modelCol.Name, effectiveAlterColumnSQLType(currentCol)),
 				})
 				if err != nil {
 					return nil, err
@@ -962,6 +964,32 @@ func (w *MigrationWriter) generateAuditScripts(schema *models.Schema, auditConfi
 
 // Helper functions for comparing database objects
 
+// renamedColumnName builds the fallback column name used when an in-place
+// type conversion fails: "<column>_<oldtype>", with the old type sanitized
+// to a valid identifier fragment (e.g. "varchar(50)" -> "varchar_50").
+func renamedColumnName(columnName, oldType string) string {
+	sanitized := strings.Map(func(r rune) rune {
+		switch {
+		case r >= 'a' && r <= 'z', r >= '0' && r <= '9':
+			return r
+		case r >= 'A' && r <= 'Z':
+			return r + ('a' - 'A')
+		default:
+			return '_'
+		}
+	}, oldType)
+
+	for strings.Contains(sanitized, "__") {
+		sanitized = strings.ReplaceAll(sanitized, "__", "_")
+	}
+	sanitized = strings.Trim(sanitized, "_")
+
+	if sanitized == "" {
+		return columnName + "_old"
+	}
+	return columnName + "_" + sanitized
+}
+
 // columnsEqual checks if two columns have the same definition
 func columnsEqual(col1, col2 *models.Column) bool {
 	if col1 == nil || col2 == nil {