add new functionality and improve backend server structure

- Enhance server functionality and optimize backend code
- Add multi-database support (PostgreSQL, MySQL, SQLite)

server/core_backend-server/tools/hook/event.go

@@ -0,0 +1,45 @@
+package hook
+
+// Resolver defines a common interface for a Hook event (see [Event]).
+type Resolver interface {
+	// Next triggers the next handler in the hook's chain (if any).
+	Next() error
+
+	// note: kept only for the generic interface; may get removed in the future
+	nextFunc() func() error
+	setNextFunc(f func() error)
+}
+
+var _ Resolver = (*Event)(nil)
+
+// Event implements [Resolver] and it is intended to be used as a base
+// Hook event that you can embed in your custom typed event structs.
+//
+// Example:
+//
+//	type CustomEvent struct {
+//		hook.Event
+//
+//		SomeField int
+//	}
+type Event struct {
+	next func() error
+}
+
+// Next calls the next hook handler.
+func (e *Event) Next() error {
+	if e.next != nil {
+		return e.next()
+	}
+	return nil
+}
+
+// nextFunc returns the function that Next calls.
+func (e *Event) nextFunc() func() error {
+	return e.next
+}
+
+// setNextFunc sets the function that Next calls.
+func (e *Event) setNextFunc(f func() error) {
+	e.next = f
+}