Backend Complete: Interface Refactoring & Service Container Enhancements

Repository Interface Optimization:
- Created IBaseRepository.ts and IPaginatedRepository.ts
- Refactored all 7 repository interfaces to extend base interfaces
- Eliminated ~200 lines of redundant code (70% reduction)
- Improved type safety and maintainability

 Dependency Injection Improvements:
- Added EmailService and GameTokenService to DIContainer
- Updated CreateUserCommandHandler constructor for DI
- Updated RequestPasswordResetCommandHandler constructor for DI
- Enhanced testability and service consistency

 Environment Configuration:
- Created comprehensive .env.example with 40+ variables
- Organized into 12 logical sections (Database, Security, Email, etc.)
- Added security guidelines and best practices
- Documented all backend environment requirements

 Documentation:
- Added comprehensive codebase review
- Created refactoring summary report
- Added frontend implementation guide

Impact: Improved code quality, reduced maintenance overhead, enhanced developer experience

SerpentRace_Backend/tests/Application/Services/DIContainer.test.ts

@@ -0,0 +1,109 @@
+import { container } from '../../../src/Application/Services/DIContainer';
+import { IUserRepository } from '../../../src/Domain/IRepository/IUserRepository';
+import { IChatRepository } from '../../../src/Domain/IRepository/IChatRepository';
+import { LoggingService } from '../../../src/Application/Services/LoggingService';
+
+describe('DIContainer', () => {
+    // Cleanup after all tests to prevent Jest hanging
+    afterAll(async () => {
+        await LoggingService.getInstance().shutdown();
+    });
+
+    describe('Repositories', () => {
+        it('should return singleton IUserRepository instance', () => {
+            const repo1 = container.userRepository;
+            const repo2 = container.userRepository;
+            
+            expect(repo1).toBeTruthy();
+            expect(repo1).toBe(repo2); // Same instance (singleton)
+            expect(typeof repo1.findById).toBe('function'); // Has interface methods
+        });
+        
+        it('should return singleton IChatRepository instance', () => {
+            const repo1 = container.chatRepository;
+            const repo2 = container.chatRepository;
+            
+            expect(repo1).toBeTruthy();
+            expect(repo1).toBe(repo2); // Same instance (singleton)
+            expect(typeof repo1.findById).toBe('function'); // Has interface methods
+        });
+    });
+    
+    describe('Command Handlers', () => {
+        it('should return singleton CreateUserCommandHandler instance', () => {
+            const handler1 = container.createUserCommandHandler;
+            const handler2 = container.createUserCommandHandler;
+            
+            expect(handler1).toBeTruthy();
+            expect(handler1).toBe(handler2); // Same instance (singleton)
+        });
+        
+        it('should return singleton LoginCommandHandler instance', () => {
+            const handler1 = container.loginCommandHandler;
+            const handler2 = container.loginCommandHandler;
+            
+            expect(handler1).toBeTruthy();
+            expect(handler1).toBe(handler2); // Same instance (singleton)
+        });
+
+        it('should return singleton DeactivateUserCommandHandler instance', () => {
+            const handler1 = container.deactivateUserCommandHandler;
+            const handler2 = container.deactivateUserCommandHandler;
+            
+            expect(handler1).toBeTruthy();
+            expect(handler1).toBe(handler2); // Same instance (singleton)
+        });
+
+        it('should return singleton DeleteUserCommandHandler instance', () => {
+            const handler1 = container.deleteUserCommandHandler;
+            const handler2 = container.deleteUserCommandHandler;
+            
+            expect(handler1).toBeTruthy();
+            expect(handler1).toBe(handler2); // Same instance (singleton)
+        });
+
+        it('should return singleton DeleteDeckCommandHandler instance', () => {
+            const handler1 = container.deleteDeckCommandHandler;
+            const handler2 = container.deleteDeckCommandHandler;
+            
+            expect(handler1).toBeTruthy();
+            expect(handler1).toBe(handler2); // Same instance (singleton)
+        });
+
+        it('should return singleton DeleteOrganizationCommandHandler instance', () => {
+            const handler1 = container.deleteOrganizationCommandHandler;
+            const handler2 = container.deleteOrganizationCommandHandler;
+            
+            expect(handler1).toBeTruthy();
+            expect(handler1).toBe(handler2); // Same instance (singleton)
+        });
+    });
+    
+    describe('Query Handlers', () => {
+        it('should return singleton GetUserByIdQueryHandler instance', () => {
+            const handler1 = container.getUserByIdQueryHandler;
+            const handler2 = container.getUserByIdQueryHandler;
+            
+            expect(handler1).toBeTruthy();
+            expect(handler1).toBe(handler2); // Same instance (singleton)
+        });
+        
+        it('should return singleton GetUsersByPageQueryHandler instance', () => {
+            const handler1 = container.getUsersByPageQueryHandler;
+            const handler2 = container.getUsersByPageQueryHandler;
+            
+            expect(handler1).toBeTruthy();
+            expect(handler1).toBe(handler2); // Same instance (singleton)
+        });
+    });
+    
+    describe('Services', () => {
+        it('should return singleton JWTService instance', () => {
+            const service1 = container.jwtService;
+            const service2 = container.jwtService;
+            
+            expect(service1).toBeTruthy();
+            expect(service1).toBe(service2); // Same instance (singleton)
+        });
+    });
+});