feat: mark generated and identity columns across writers and DBML

- bun: scanonly+generated for GENERATED columns; scanonly+identity for ALWAYS identity (PK gets identity only)
- gorm: <-:false+generated / identity, same primary key rule
- dbml: write GENERATED/IDENTITY column notes and read them back into the model
- pgsql: emit generation/identity clauses in migration create-table and add-column, no DEFAULT
- readmes updated

pkg/writers/pgsql/templates.go

@@ -77,6 +77,10 @@ type ColumnData struct {
 	Type    string
 	Default string
 	NotNull bool
+	// Generated is the expression of a GENERATED ALWAYS AS (...) STORED column.
+	Generated string
+	// Identity is "ALWAYS" or "BY DEFAULT" for a GENERATED ... AS IDENTITY column.
+	Identity string
 }
 
 // AddColumnData contains data for add column template
@@ -87,6 +91,10 @@ type AddColumnData struct {
 	ColumnType string
 	Default    string
 	NotNull    bool
+	// Generated is the expression of a GENERATED ALWAYS AS (...) STORED column.
+	Generated string
+	// Identity is "ALWAYS" or "BY DEFAULT" for a GENERATED ... AS IDENTITY column.
+	Identity string
 }
 
 // AlterColumnTypeWithCheckData contains data for the guarded alter column
@@ -520,6 +528,19 @@ func (te *TemplateExecutor) ExecuteCreatePrimaryKeyWithAutoGenCheck(data CreateP
 
 // Helper functions to build template data from models
 
+// columnGenerationClauses returns the generation expression of a GENERATED ... STORED
+// column and the generation mode of an identity column ("" for each that does not apply).
+// Such columns cannot carry a DEFAULT.
+func columnGenerationClauses(col *models.Column) (generated, identity string) {
+	if col.Generated && col.GenerationExpression != "" {
+		return col.GenerationExpression, ""
+	}
+	if col.Identity && col.IdentityGeneration != "" {
+		return "", strings.ToUpper(col.IdentityGeneration)
+	}
+	return "", ""
+}
+
 // BuildCreateTableData builds CreateTableData from a models.Table
 func BuildCreateTableData(schemaName string, table *models.Table) CreateTableData {
 	columns := make([]ColumnData, 0, len(table.Columns))
@@ -532,7 +553,8 @@ func BuildCreateTableData(schemaName string, table *models.Table) CreateTableDat
 			Type:    col.Type,
 			NotNull: col.NotNull,
 		}
-		if col.Default != nil {
+		colData.Generated, colData.Identity = columnGenerationClauses(col)
+		if col.Default != nil && colData.Generated == "" && colData.Identity == "" {
 			if value, ok := col.Default.(string); ok {
 				colData.Default = writers.QuoteDefaultValue(stripBackticks(value), col.Type)
 			} else {