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/writer.go

@@ -475,6 +475,75 @@ func (w *Writer) GenerateAlterColumnTypeStatements(schema *models.Schema) ([]str
 	return statements, nil
 }
 
+// GenerateAlterColumnDefaultStatements generates guarded ALTER TABLE
+// statements to bring existing columns' DEFAULT clause in line with the
+// model, safe to run against a database that already has the columns.
+func (w *Writer) GenerateAlterColumnDefaultStatements(schema *models.Schema) ([]string, error) {
+	statements := []string{}
+
+	statements = append(statements, fmt.Sprintf("-- Alter column defaults for schema: %s", schema.Name))
+
+	for _, table := range schema.Tables {
+		columns := getSortedColumns(table.Columns)
+		for _, col := range columns {
+			setDefault, defaultVal := formatColumnDefaultSQL(col)
+			stmt, err := w.executor.ExecuteAlterColumnDefaultWithCheck(AlterColumnDefaultWithCheckData{
+				SchemaName:   schema.Name,
+				TableName:    table.Name,
+				ColumnName:   col.Name,
+				SetDefault:   setDefault,
+				DefaultValue: defaultVal,
+			})
+			if err != nil {
+				return nil, fmt.Errorf("failed to generate alter column default for %s.%s.%s: %w", schema.Name, table.Name, col.Name, err)
+			}
+			statements = append(statements, stmt)
+		}
+	}
+
+	return statements, nil
+}
+
+// formatColumnDefaultSQL renders a column's model-level default into the
+// SQL literal/expression used by ALTER COLUMN ... SET DEFAULT, shared by
+// the full-schema writer and the diff-based migration writer.
+func formatColumnDefaultSQL(col *models.Column) (setDefault bool, defaultVal string) {
+	if col.Default == nil {
+		return false, ""
+	}
+	if value, ok := col.Default.(string); ok {
+		return true, writers.QuoteDefaultValue(value, col.Type)
+	}
+	return true, fmt.Sprintf("%v", col.Default)
+}
+
+// GenerateAlterColumnNullabilityStatements generates guarded ALTER TABLE
+// statements to bring existing columns' NOT NULL state in line with the
+// model, safe to run against a database that already has the columns.
+func (w *Writer) GenerateAlterColumnNullabilityStatements(schema *models.Schema) ([]string, error) {
+	statements := []string{}
+
+	statements = append(statements, fmt.Sprintf("-- Alter column nullability for schema: %s", schema.Name))
+
+	for _, table := range schema.Tables {
+		columns := getSortedColumns(table.Columns)
+		for _, col := range columns {
+			stmt, err := w.executor.ExecuteAlterColumnNullabilityWithCheck(AlterColumnNullabilityWithCheckData{
+				SchemaName: schema.Name,
+				TableName:  table.Name,
+				ColumnName: col.Name,
+				NotNull:    col.NotNull,
+			})
+			if err != nil {
+				return nil, fmt.Errorf("failed to generate alter column nullability for %s.%s.%s: %w", schema.Name, table.Name, col.Name, err)
+			}
+			statements = append(statements, stmt)
+		}
+	}
+
+	return statements, nil
+}
+
 // GenerateAddColumnsForDatabase generates ALTER TABLE ADD COLUMN statements for the entire database
 func (w *Writer) GenerateAddColumnsForDatabase(db *models.Database) ([]string, error) {
 	statements := []string{}
@@ -641,6 +710,14 @@ func (w *Writer) WriteSchema(schema *models.Schema) error {
 		return err
 	}
 
+	if err := w.writeAlterColumnDefaults(schema); err != nil {
+		return err
+	}
+
+	if err := w.writeAlterColumnNullability(schema); err != nil {
+		return err
+	}
+
 	// Phase 4: Create primary keys (priority 160)
 	if err := w.writePrimaryKeys(schema); err != nil {
 		return err
@@ -859,6 +936,36 @@ func (w *Writer) writeAlterColumnTypes(schema *models.Schema) error {
 	return nil
 }
 
+func (w *Writer) writeAlterColumnDefaults(schema *models.Schema) error {
+	fmt.Fprintf(w.writer, "-- Alter column defaults for schema: %s\n", schema.Name)
+
+	statements, err := w.GenerateAlterColumnDefaultStatements(schema)
+	if err != nil {
+		return err
+	}
+	for _, stmt := range statements[1:] {
+		fmt.Fprint(w.writer, stmt)
+		fmt.Fprint(w.writer, "\n")
+	}
+
+	return nil
+}
+
+func (w *Writer) writeAlterColumnNullability(schema *models.Schema) error {
+	fmt.Fprintf(w.writer, "-- Alter column nullability for schema: %s\n", schema.Name)
+
+	statements, err := w.GenerateAlterColumnNullabilityStatements(schema)
+	if err != nil {
+		return err
+	}
+	for _, stmt := range statements[1:] {
+		fmt.Fprint(w.writer, stmt)
+		fmt.Fprint(w.writer, "\n")
+	}
+
+	return nil
+}
+
 // writePrimaryKeys generates ALTER TABLE statements for primary keys
 func (w *Writer) writePrimaryKeys(schema *models.Schema) error {
 	fmt.Fprintf(w.writer, "-- Primary keys for schema: %s\n", schema.Name)