feat(pgsql): add support for concurrent index creation

* Implemented `Concurrent` field in index model
* Updated index creation template to support `CREATE INDEX CONCURRENTLY`
* Added tests for concurrent index creation in migration writer

pkg/writers/pgsql/migration_writer_test.go

@@ -334,6 +334,46 @@ func TestWriteMigration_DoesNotAlterEquivalentNormalizedColumnType(t *testing.T)
 	}
 }
 
+func TestWriteMigration_ConcurrentIndex(t *testing.T) {
+	current := models.InitDatabase("testdb")
+	currentSchema := models.InitSchema("public")
+	current.Schemas = append(current.Schemas, currentSchema)
+
+	model := models.InitDatabase("testdb")
+	modelSchema := models.InitSchema("public")
+
+	table := models.InitTable("articles", "public")
+	titleCol := models.InitColumn("title", "articles", "public")
+	titleCol.Type = "text"
+	table.Columns["title"] = titleCol
+
+	index := &models.Index{
+		Name:       "idx_articles_title",
+		Columns:    []string{"title"},
+		Concurrent: true,
+	}
+	table.Indexes[index.Name] = index
+
+	modelSchema.Tables = append(modelSchema.Tables, table)
+	model.Schemas = append(model.Schemas, modelSchema)
+
+	var buf bytes.Buffer
+	writer, err := NewMigrationWriter(&writers.WriterOptions{})
+	if err != nil {
+		t.Fatalf("Failed to create writer: %v", err)
+	}
+	writer.writer = &buf
+
+	if err := writer.WriteMigration(model, current); err != nil {
+		t.Fatalf("WriteMigration failed: %v", err)
+	}
+
+	output := buf.String()
+	if !strings.Contains(output, "CREATE INDEX CONCURRENTLY IF NOT EXISTS") {
+		t.Fatalf("expected CONCURRENTLY create index statement, got:\n%s", output)
+	}
+}
+
 func TestWriteMigration_GinIndexOnTextUsesTrigramOperatorClass(t *testing.T) {
 	current := models.InitDatabase("testdb")
 	currentSchema := models.InitSchema("public")