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/README.md

@@ -169,6 +169,7 @@ When `include_audit` is enabled, adds:
 - Constraint actions (CASCADE, RESTRICT, SET NULL)
 - Partial indexes
 - Function-based indexes
+- Concurrent index creation (`CREATE INDEX CONCURRENTLY`) via `Index.Concurrent`
 - Check constraints with expressions
 
 ## Data Types

pkg/writers/pgsql/migration_writer.go

@@ -652,6 +652,7 @@ func (w *MigrationWriter) generateIndexScripts(model *models.Schema, current *mo
 					IndexType:  indexType,
 					Columns:    strings.Join(columnExprs, ", "),
 					Unique:     modelIndex.Unique,
+					Concurrent: modelIndex.Concurrent,
 				})
 				if err != nil {
 					return nil, err

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")

pkg/writers/pgsql/templates.go

@@ -139,6 +139,7 @@ type CreateIndexData struct {
 	IndexType  string
 	Columns    string
 	Unique     bool
+	Concurrent bool
 }
 
 // CreateForeignKeyData contains data for create foreign key template

pkg/writers/pgsql/templates/create_index.tmpl

@@ -1,2 +1,2 @@
-CREATE {{if .Unique}}UNIQUE {{end}}INDEX IF NOT EXISTS {{quote_ident .IndexName}}
+CREATE {{if .Unique}}UNIQUE {{end}}INDEX {{if .Concurrent}}CONCURRENTLY {{end}}IF NOT EXISTS {{quote_ident .IndexName}}
   ON {{qual_table .SchemaName .TableName}} USING {{.IndexType}} ({{.Columns}});

pkg/writers/pgsql/writer.go

@@ -1097,8 +1097,13 @@ func (w *Writer) writeIndexes(schema *models.Schema) error {
 				whereClause = fmt.Sprintf(" WHERE %s", index.Where)
 			}
 
-			fmt.Fprintf(w.writer, "CREATE %sINDEX IF NOT EXISTS %s\n",
-				unique, indexName)
+			concurrently := ""
+			if index.Concurrent {
+				concurrently = "CONCURRENTLY "
+			}
+
+			fmt.Fprintf(w.writer, "CREATE %sINDEX %sIF NOT EXISTS %s\n",
+				unique, concurrently, indexName)
 			fmt.Fprintf(w.writer, "  ON %s USING %s (%s)%s;\n\n",
 				w.qualTable(schema.SQLName(), table.SQLName()), indexType, strings.Join(columnExprs, ", "), whereClause)
 		}

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")