feat(relations): 🎉 add flatten schema option for output

* Introduce `--flatten-schema` flag to convert, merge, and split commands.
* Modify database writing functions to support flattened schema names.
* Update template functions to handle schema.table naming convention.
* Enhance PostgreSQL writer to utilize flattened schema in generated SQL.
* Update tests to ensure compatibility with new flattening feature.
* Dependencies updated for improved functionality.

cmd/relspec/convert.go

@@ -38,13 +38,14 @@ import (
 )
 
 var (
-	convertSourceType   string
-	convertSourcePath   string
-	convertSourceConn   string
-	convertTargetType   string
-	convertTargetPath   string
-	convertPackageName  string
-	convertSchemaFilter string
+	convertSourceType    string
+	convertSourcePath    string
+	convertSourceConn    string
+	convertTargetType    string
+	convertTargetPath    string
+	convertPackageName   string
+	convertSchemaFilter  string
+	convertFlattenSchema bool
 )
 
 var convertCmd = &cobra.Command{
@@ -148,6 +149,7 @@ func init() {
 	convertCmd.Flags().StringVar(&convertTargetPath, "to-path", "", "Target output path (file or directory)")
 	convertCmd.Flags().StringVar(&convertPackageName, "package", "", "Package name (for code generation formats like gorm/bun)")
 	convertCmd.Flags().StringVar(&convertSchemaFilter, "schema", "", "Filter to a specific schema by name (required for formats like dctx that only support single schemas)")
+	convertCmd.Flags().BoolVar(&convertFlattenSchema, "flatten-schema", false, "Flatten schema.table names to schema_table (useful for databases like SQLite that do not support schemas)")
 
 	err := convertCmd.MarkFlagRequired("from")
 	if err != nil {
@@ -202,7 +204,7 @@ func runConvert(cmd *cobra.Command, args []string) error {
 		fmt.Fprintf(os.Stderr, "      Schema: %s\n", convertSchemaFilter)
 	}
 
-	if err := writeDatabase(db, convertTargetType, convertTargetPath, convertPackageName, convertSchemaFilter); err != nil {
+	if err := writeDatabase(db, convertTargetType, convertTargetPath, convertPackageName, convertSchemaFilter, convertFlattenSchema); err != nil {
 		return fmt.Errorf("failed to write target: %w", err)
 	}
 
@@ -301,12 +303,13 @@ func readDatabaseForConvert(dbType, filePath, connString string) (*models.Databa
 	return db, nil
 }
 
-func writeDatabase(db *models.Database, dbType, outputPath, packageName, schemaFilter string) error {
+func writeDatabase(db *models.Database, dbType, outputPath, packageName, schemaFilter string, flattenSchema bool) error {
 	var writer writers.Writer
 
 	writerOpts := &writers.WriterOptions{
-		OutputPath:  outputPath,
-		PackageName: packageName,
+		OutputPath:    outputPath,
+		PackageName:   packageName,
+		FlattenSchema: flattenSchema,
 	}
 
 	switch strings.ToLower(dbType) {