Implement serialization/deserialization and persistent storage across services

- Add serialize/deserialize support for core models
- Add file-based load/save functions in management services
- Introduce FileManager, Config, Utility and helper utilities
- Persist observer IDs for notification services
- Resolve object relationships during load (services, bookings, invoices, job cards)
- Add controller-level loadSystemData/saveSystemData
- Load data at app startup and save on shutdown

Trenser.VehicleServiceSystem/Trenser.VehicleServiceSystem/models/InventoryItem.h

@@ -14,6 +14,7 @@ private:
 public:
     InventoryItem();
     InventoryItem(const std::string& partName, int quantity, double price);
+    InventoryItem(const std::string& id, const std::string& partName, int quantity, double price, util::State status);
     const std::string& getId() const;
     const std::string& getPartName() const;
     int getQuantity() const;
@@ -24,4 +25,7 @@ public:
     void setQuantity(int quantity);
     void setPrice(double price);
     void setState(util::State status);
+    std::string serialize() const;
+    static InventoryItem* deserialize(const std::string&);
+    static std::string getHeaders();
 };