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/services/InventoryManagementService.h

@@ -10,6 +10,7 @@ class InventoryManagementService : public NotificationManagementService
 {
 private:
     DataStore& m_dataStore;
+    static util::Map<std::string, User*> m_observers;
 public:
     InventoryManagementService() : m_dataStore(DataStore::getInstance()) {}
     util::Map<std::string, InventoryItem*> getInventoryItems();
@@ -17,5 +18,7 @@ public:
     void addInventoryItem(const std::string& partName, int quantity, double price);
     void removeInventoryItem(const std::string& inventoryItemID);
     void sendLowStockAlerts();
-    void sendNotification(User* user, const std::string& title, const std::string& message);
+    void sendNotification(User* user, const std::string& title, const std::string& message) override;
+    void attach(User* user) override;
+    void detach(User* user) override;
 };