fix(pgsql): handle nullability/type/default drift on existing columns

Existing databases that already ran an old migration kept stale NOT
NULL constraints and mismatched column types/defaults, because the
schema writer only emitted idempotent ADD COLUMN IF NOT EXISTS guards
and never altered columns that already existed.

- Emit guarded ALTER COLUMN ... SET/DROP NOT NULL when a column's
  nullability differs from the model.
- Emit guarded ALTER COLUMN ... TYPE, falling back to renaming the old
  column and adding a fresh one when the in-place conversion fails.
- Emit guarded ALTER COLUMN ... SET/DROP DEFAULT for default drift.
- Collapse the previously duplicated plain/guarded templates so
  WriteSchema (full-schema, live-state-checking) and WriteMigration
  (diff-based) share the same guarded SQL templates and Go helpers
  instead of maintaining the logic twice.

pkg/writers/pgsql/migration_writer.go

@@ -409,14 +409,7 @@ func (w *MigrationWriter) generateAlterTableScripts(schema *models.Schema, model
 
 		if !exists {
 			// Column doesn't exist, add it
-			defaultVal := ""
-			if modelCol.Default != nil {
-				if value, ok := modelCol.Default.(string); ok {
-					defaultVal = writers.QuoteDefaultValue(value, modelCol.Type)
-				} else {
-					defaultVal = fmt.Sprintf("%v", modelCol.Default)
-				}
-			}
+			_, defaultVal := formatColumnDefaultSQL(modelCol)
 
 			sql, err := w.executor.ExecuteAddColumn(AddColumnData{
 				SchemaName: schema.Name,
@@ -443,13 +436,13 @@ func (w *MigrationWriter) generateAlterTableScripts(schema *models.Schema, model
 			// Column exists but properties changed
 			if !columnTypesEqual(modelCol, currentCol) {
 				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)),
+				sql, err := w.executor.ExecuteAlterColumnTypeWithCheck(AlterColumnTypeWithCheckData{
+					SchemaName:      schema.Name,
+					TableName:       modelTable.Name,
+					ColumnName:      modelCol.Name,
+					NewType:         newType,
+					EquivalentTypes: equivalentTypeListSQL(newType),
+					UsingExpr:       buildAlterColumnUsingExpression(modelCol.Name, newType),
 				})
 				if err != nil {
 					return nil, err
@@ -468,17 +461,9 @@ func (w *MigrationWriter) generateAlterTableScripts(schema *models.Schema, model
 
 			// Check default value changes
 			if fmt.Sprintf("%v", modelCol.Default) != fmt.Sprintf("%v", currentCol.Default) {
-				setDefault := modelCol.Default != nil
-				defaultVal := ""
-				if setDefault {
-					if value, ok := modelCol.Default.(string); ok {
-						defaultVal = writers.QuoteDefaultValue(value, modelCol.Type)
-					} else {
-						defaultVal = fmt.Sprintf("%v", modelCol.Default)
-					}
-				}
+				setDefault, defaultVal := formatColumnDefaultSQL(modelCol)
 
-				sql, err := w.executor.ExecuteAlterColumnDefault(AlterColumnDefaultData{
+				sql, err := w.executor.ExecuteAlterColumnDefaultWithCheck(AlterColumnDefaultWithCheckData{
 					SchemaName:   schema.Name,
 					TableName:    modelTable.Name,
 					ColumnName:   modelCol.Name,
@@ -502,7 +487,7 @@ func (w *MigrationWriter) generateAlterTableScripts(schema *models.Schema, model
 
 			// Check nullability changes
 			if modelCol.NotNull != currentCol.NotNull {
-				sql, err := w.executor.ExecuteAlterColumnNullability(AlterColumnNullabilityData{
+				sql, err := w.executor.ExecuteAlterColumnNullabilityWithCheck(AlterColumnNullabilityWithCheckData{
 					SchemaName: schema.Name,
 					TableName:  modelTable.Name,
 					ColumnName: modelCol.Name,
@@ -964,32 +949,6 @@ 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 {