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.

pkg/writers/writer.go

@@ -28,10 +28,29 @@ type WriterOptions struct {
 	// PackageName is the Go package name (for code generation)
 	PackageName string
 
+	// FlattenSchema disables schema.table dot notation and instead joins
+	// schema and table with an underscore (e.g., "public_users").
+	// Useful for databases like SQLite that do not support schemas.
+	FlattenSchema bool
+
 	// Additional options can be added here as needed
 	Metadata map[string]interface{}
 }
 
+// QualifiedTableName returns a schema-qualified table name.
+// When flatten is true, schema and table are joined with underscore (e.g., "schema_table").
+// When flatten is false, they are dot-separated (e.g., "schema.table").
+// If schema is empty, just the table name is returned regardless of flatten.
+func QualifiedTableName(schema, table string, flatten bool) string {
+	if schema == "" {
+		return table
+	}
+	if flatten {
+		return schema + "_" + table
+	}
+	return schema + "." + table
+}
+
 // SanitizeFilename removes quotes, comments, and invalid characters from identifiers
 // to make them safe for use in filenames. This handles:
 // - Double and single quotes: "table_name" or 'table_name' -> table_name