Refactor backend services and controllers to use DTOs for Tenant, Lease, and Apartment

- Updated TenantController to utilize TenantService and DTOs for tenant operations.
- Refactored UserController to return UserResponse DTO.
- Introduced LeaseService and ApartmentService to handle business logic and data mapping.
- Created DTOs for Apartment, Lease, Tenant, and User to standardize data transfer.
- Implemented mappers for converting between entities and DTOs using MapStruct.
- Removed landlordId from Apartment and Lease entities and adjusted related tests.
- Added validation and normalization logic for tenant and guarantor data.
- Enhanced unit tests to ensure proper mapping and data integrity during updates.

frontend/src/app/services/session.service.ts

@@ -1,6 +1,12 @@
 import { HttpClient } from '@angular/common/http';
 import { Injectable, inject, signal } from '@angular/core';
 
+interface UserResponse {
+  authenticated: boolean;
+  username?: string;
+  email?: string;
+}
+
 @Injectable({ providedIn: 'root' })
 export class SessionService {
   private readonly http = inject(HttpClient);
@@ -9,7 +15,7 @@ export class SessionService {
   readonly email = signal('');
 
   refresh(): void {
-    this.http.get<{ authenticated: boolean; username?: string; email?: string }>('/api/user').subscribe({
+    this.http.get<UserResponse>('/api/user').subscribe({
       next: (user) => {
         this.isLoggedIn.set(user.authenticated);
         this.username.set(user.authenticated ? (user.username || 'User') : '');