feat: Add initial backend server with user authentication, session management, and playlist CRUD functionality.

lib/core/services/auth_api_service.dart

@@ -0,0 +1,98 @@
+import '../api/api_client.dart';
+import '../models/app_user.dart';
+
+/// Service for authentication via API
+class AuthApiService {
+  final ApiClient _api = ApiClient();
+
+  /// Login with username and password
+  Future<AuthResult> login(String username, String password) async {
+    try {
+      final response = await _api.post('/api/auth/login', data: {
+        'username': username,
+        'password': password,
+      });
+
+      final data = response.data as Map<String, dynamic>;
+      
+      if (data['success'] == true) {
+        final token = data['token'] as String;
+        final userData = data['user'] as Map<String, dynamic>;
+        
+        _api.setToken(token);
+        
+        return AuthResult(
+          success: true,
+          user: AppUser(
+            id: userData['id'] as String,
+            username: userData['username'] as String,
+            passwordHash: '', // Not needed for API auth
+            isAdmin: userData['isAdmin'] as bool? ?? false,
+            createdAt: DateTime.now(),
+          ),
+          token: token,
+        );
+      } else {
+        return AuthResult(
+          success: false,
+          error: data['error'] as String? ?? 'Login failed',
+        );
+      }
+    } catch (e) {
+      return AuthResult(
+        success: false,
+        error: 'Network error: $e',
+      );
+    }
+  }
+
+  /// Logout
+  Future<void> logout() async {
+    try {
+      await _api.post('/api/auth/logout');
+    } finally {
+      _api.clearToken();
+    }
+  }
+
+  /// Get current user from token
+  Future<AppUser?> getCurrentUser() async {
+    _api.restoreToken();
+    
+    if (_api.getStoredToken() == null) {
+      return null;
+    }
+
+    try {
+      final response = await _api.get('/api/auth/me');
+      final data = response.data as Map<String, dynamic>;
+      final userData = data['user'] as Map<String, dynamic>;
+      
+      return AppUser(
+        id: userData['id'] as String,
+        username: userData['username'] as String,
+        passwordHash: '',
+        isAdmin: userData['isAdmin'] as bool? ?? false,
+        createdAt: DateTime.now(),
+      );
+    } catch (e) {
+      _api.clearToken();
+      return null;
+    }
+  }
+}
+
+/// Result of authentication attempt
+class AuthResult {
+  final bool success;
+  final AppUser? user;
+  final String? token;
+  final String? error;
+
+  AuthResult({
+    required this.success,
+    this.user,
+    this.token,
+    this.error,
+  });
+}