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/writer_test.go

@@ -87,6 +87,41 @@ func TestWriteDatabase(t *testing.T) {
 	}
 }
 
+func TestWriteDatabase_ConcurrentIndex(t *testing.T) {
+	db := models.InitDatabase("testdb")
+	schema := models.InitSchema("public")
+
+	table := models.InitTable("users", "public")
+
+	emailCol := models.InitColumn("email", "users", "public")
+	emailCol.Type = "text"
+	table.Columns["email"] = emailCol
+
+	concurrentIndex := &models.Index{
+		Name:       "idx_users_email",
+		Columns:    []string{"email"},
+		Concurrent: true,
+	}
+	table.Indexes["idx_users_email"] = concurrentIndex
+
+	schema.Tables = append(schema.Tables, table)
+	db.Schemas = append(db.Schemas, schema)
+
+	var buf bytes.Buffer
+	writer := NewWriter(&writers.WriterOptions{})
+	writer.writer = &buf
+
+	if err := writer.WriteDatabase(db); err != nil {
+		t.Fatalf("WriteDatabase failed: %v", err)
+	}
+
+	output := buf.String()
+
+	if !strings.Contains(output, "CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_users_email") {
+		t.Errorf("Output missing CONCURRENTLY index creation:\n%s", output)
+	}
+}
+
 func TestWriteDatabase_GinIndexOnTextArrayDoesNotUseTrigramOperatorClass(t *testing.T) {
 	db := models.InitDatabase("testdb")
 	schema := models.InitSchema("public")