Refactor notification service abstraction

- Removed observer map from Subject base class
- Removed notify() from Subject interface
- Made NotificationManagementService abstract with pure virtual methods
- Added static observer maps to notification service implementations
- Added attach() and detach() overrides in inventory, payment, and service management services
- Added update() override in User model for observer pattern support
- Updated notification method declarations with override specifiers

Trenser.VehicleServiceSystem/Trenser.VehicleServiceSystem/core/patterns/Subject.h

@@ -7,11 +7,8 @@ class Notification;
 
 class Subject
 {
-protected:
-    util::Map<std::string, User*> m_observers;
 public:
     virtual ~Subject() = default;
     virtual void attach(User* user) = 0;
     virtual void detach(User* user) = 0;
-    virtual void notify(Notification* notification) = 0;
 };