add changes

backend/config/config.js

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+require('dotenv').config();
+
+/**
+ * Application Configuration
+ * Centralized configuration management for all environment variables
+ */
+
+const config = {
+  // Server Configuration
+  server: {
+    nodeEnv: process.env.NODE_ENV || 'development',
+    port: parseInt(process.env.PORT) || 3000,
+    apiPrefix: process.env.API_PREFIX || '/api',
+    isDevelopment: (process.env.NODE_ENV || 'development') === 'development',
+    isProduction: process.env.NODE_ENV === 'production',
+    isTest: process.env.NODE_ENV === 'test'
+  },
+
+  // Database Configuration
+  database: {
+    host: process.env.DB_HOST || 'localhost',
+    port: parseInt(process.env.DB_PORT) || 3306,
+    name: process.env.DB_NAME || 'interview_quiz_db',
+    user: process.env.DB_USER || 'root',
+    password: process.env.DB_PASSWORD || '',
+    dialect: process.env.DB_DIALECT || 'mysql',
+    pool: {
+      max: parseInt(process.env.DB_POOL_MAX) || 10,
+      min: parseInt(process.env.DB_POOL_MIN) || 0,
+      acquire: parseInt(process.env.DB_POOL_ACQUIRE) || 30000,
+      idle: parseInt(process.env.DB_POOL_IDLE) || 10000
+    }
+  },
+
+  // JWT Configuration
+  jwt: {
+    secret: process.env.JWT_SECRET,
+    expire: process.env.JWT_EXPIRE || '24h',
+    algorithm: 'HS256'
+  },
+
+  // Rate Limiting Configuration
+  rateLimit: {
+    windowMs: parseInt(process.env.RATE_LIMIT_WINDOW_MS) || 900000, // 15 minutes
+    maxRequests: parseInt(process.env.RATE_LIMIT_MAX_REQUESTS) || 100,
+    message: 'Too many requests from this IP, please try again later.'
+  },
+
+  // CORS Configuration
+  cors: {
+    origin: process.env.CORS_ORIGIN || 'http://localhost:4200',
+    credentials: true
+  },
+
+  // Guest Session Configuration
+  guest: {
+    sessionExpireHours: parseInt(process.env.GUEST_SESSION_EXPIRE_HOURS) || 24,
+    maxQuizzes: parseInt(process.env.GUEST_MAX_QUIZZES) || 3
+  },
+
+  // Logging Configuration
+  logging: {
+    level: process.env.LOG_LEVEL || 'info'
+  },
+
+  // Pagination Defaults
+  pagination: {
+    defaultLimit: 10,
+    maxLimit: 100
+  },
+
+  // Security Configuration
+  security: {
+    bcryptRounds: 10,
+    maxLoginAttempts: 5,
+    lockoutDuration: 15 * 60 * 1000 // 15 minutes
+  }
+};
+
+/**
+ * Validate critical configuration values
+ */
+function validateConfig() {
+  const errors = [];
+
+  if (!config.jwt.secret) {
+    errors.push('JWT_SECRET is not configured');
+  }
+
+  if (!config.database.name) {
+    errors.push('DB_NAME is not configured');
+  }
+
+  if (config.server.isProduction && !config.database.password) {
+    errors.push('DB_PASSWORD is required in production');
+  }
+
+  if (errors.length > 0) {
+    throw new Error(`Configuration errors:\n  - ${errors.join('\n  - ')}`);
+  }
+
+  return true;
+}
+
+// Validate on module load
+try {
+  validateConfig();
+} catch (error) {
+  console.error('❌ Configuration Error:', error.message);
+  process.exit(1);
+}
+
+module.exports = config;

backend/config/database.js

@@ -0,0 +1,76 @@
+require('dotenv').config();
+
+module.exports = {
+  development: {
+    username: process.env.DB_USER || 'root',
+    password: process.env.DB_PASSWORD || '',
+    database: process.env.DB_NAME || 'interview_quiz_db',
+    host: process.env.DB_HOST || 'localhost',
+    port: process.env.DB_PORT || 3306,
+    dialect: process.env.DB_DIALECT || 'mysql',
+    logging: console.log,
+    pool: {
+      max: parseInt(process.env.DB_POOL_MAX) || 10,
+      min: parseInt(process.env.DB_POOL_MIN) || 0,
+      acquire: parseInt(process.env.DB_POOL_ACQUIRE) || 30000,
+      idle: parseInt(process.env.DB_POOL_IDLE) || 10000
+    },
+    define: {
+      timestamps: true,
+      underscored: true,
+      freezeTableName: false,
+      charset: 'utf8mb4',
+      collate: 'utf8mb4_unicode_ci'
+    }
+  },
+  test: {
+    username: process.env.DB_USER || 'root',
+    password: process.env.DB_PASSWORD || '',
+    database: process.env.DB_NAME + '_test' || 'interview_quiz_db_test',
+    host: process.env.DB_HOST || 'localhost',
+    port: process.env.DB_PORT || 3306,
+    dialect: process.env.DB_DIALECT || 'mysql',
+    logging: false,
+    pool: {
+      max: 5,
+      min: 0,
+      acquire: 30000,
+      idle: 10000
+    },
+    define: {
+      timestamps: true,
+      underscored: true,
+      freezeTableName: false,
+      charset: 'utf8mb4',
+      collate: 'utf8mb4_unicode_ci'
+    }
+  },
+  production: {
+    username: process.env.DB_USER,
+    password: process.env.DB_PASSWORD,
+    database: process.env.DB_NAME,
+    host: process.env.DB_HOST,
+    port: process.env.DB_PORT || 3306,
+    dialect: process.env.DB_DIALECT || 'mysql',
+    logging: false,
+    pool: {
+      max: parseInt(process.env.DB_POOL_MAX) || 20,
+      min: parseInt(process.env.DB_POOL_MIN) || 5,
+      acquire: parseInt(process.env.DB_POOL_ACQUIRE) || 30000,
+      idle: parseInt(process.env.DB_POOL_IDLE) || 10000
+    },
+    define: {
+      timestamps: true,
+      underscored: true,
+      freezeTableName: false,
+      charset: 'utf8mb4',
+      collate: 'utf8mb4_unicode_ci'
+    },
+    dialectOptions: {
+      ssl: {
+        require: true,
+        rejectUnauthorized: false
+      }
+    }
+  }
+};

backend/config/db.js

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+const db = require('../models');
+
+/**
+ * Test database connection
+ */
+async function testConnection() {
+  try {
+    await db.sequelize.authenticate();
+    console.log('✅ Database connection verified');
+    return true;
+  } catch (error) {
+    console.error('❌ Database connection failed:', error.message);
+    return false;
+  }
+}
+
+/**
+ * Sync all models with database
+ * WARNING: Use with caution in production
+ */
+async function syncModels(options = {}) {
+  try {
+    await db.sequelize.sync(options);
+    console.log('✅ Models synchronized with database');
+    return true;
+  } catch (error) {
+    console.error('❌ Model synchronization failed:', error.message);
+    return false;
+  }
+}
+
+/**
+ * Close database connection
+ */
+async function closeConnection() {
+  try {
+    await db.sequelize.close();
+    console.log('✅ Database connection closed');
+    return true;
+  } catch (error) {
+    console.error('❌ Failed to close database connection:', error.message);
+    return false;
+  }
+}
+
+/**
+ * Get database statistics
+ */
+async function getDatabaseStats() {
+  try {
+    const [tables] = await db.sequelize.query('SHOW TABLES');
+    const [version] = await db.sequelize.query('SELECT VERSION() as version');
+    
+    return {
+      connected: true,
+      version: version[0].version,
+      tables: tables.length,
+      database: db.sequelize.config.database
+    };
+  } catch (error) {
+    return {
+      connected: false,
+      error: error.message
+    };
+  }
+}
+
+module.exports = {
+  db,
+  testConnection,
+  syncModels,
+  closeConnection,
+  getDatabaseStats
+};