Add comprehensive database setup and user management system

- Implement PostgreSQL database schema with users and bookmarks tables
- Add database connection pooling with retry logic and error handling
- Create migration system with automatic schema initialization
- Add database CLI tools for management (init, status, validate, etc.)
- Include comprehensive error handling and diagnostics
- Add development seed data and testing utilities
- Implement health monitoring and connection pool statistics
- Create detailed documentation and troubleshooting guide

Database features:
- Users table with authentication fields and email verification
- Bookmarks table with user association and metadata
- Proper indexes for performance optimization
- Automatic timestamp triggers
- Transaction support with rollback handling
- Connection pooling (20 max connections, 30s idle timeout)
- Graceful shutdown handling

CLI commands available:
- npm run db:init - Initialize database
- npm run db:status - Check database status
- npm run db:validate - Validate schema
- npm run db:test - Run database tests
- npm run db:diagnostics - Full diagnostics

backend/src/app.js

@@ -0,0 +1,113 @@
+const express = require('express');
+const helmet = require('helmet');
+const rateLimit = require('express-rate-limit');
+const cookieParser = require('cookie-parser');
+require('dotenv').config();
+
+const app = express();
+
+// Security middleware
+app.use(helmet());
+
+// Rate limiting
+const limiter = rateLimit({
+  windowMs: 15 * 60 * 1000, // 15 minutes
+  max: 100, // limit each IP to 100 requests per windowMs
+  message: 'Too many requests from this IP, please try again later.'
+});
+app.use(limiter);
+
+// Body parsing middleware
+app.use(express.json({ limit: '10mb' }));
+app.use(express.urlencoded({ extended: true }));
+app.use(cookieParser());
+
+// CORS middleware
+app.use((req, res, next) => {
+  const allowedOrigins = process.env.ALLOWED_ORIGINS?.split(',') || ['http://localhost:3000'];
+  const origin = req.headers.origin;
+  
+  if (allowedOrigins.includes(origin)) {
+    res.setHeader('Access-Control-Allow-Origin', origin);
+  }
+  
+  res.setHeader('Access-Control-Allow-Methods', 'GET, POST, PUT, DELETE, OPTIONS');
+  res.setHeader('Access-Control-Allow-Headers', 'Content-Type, Authorization');
+  res.setHeader('Access-Control-Allow-Credentials', 'true');
+  
+  if (req.method === 'OPTIONS') {
+    res.sendStatus(200);
+  } else {
+    next();
+  }
+});
+
+// Health check endpoint
+app.get('/health', async (req, res) => {
+  try {
+    const dbConnection = require('./database/connection');
+    const dbUtils = require('./database/utils');
+    
+    const health = await dbConnection.healthCheck();
+    const diagnostics = await dbUtils.diagnostics();
+    
+    res.json({ 
+      status: health.healthy ? 'OK' : 'ERROR',
+      timestamp: new Date().toISOString(),
+      database: health,
+      diagnostics: diagnostics
+    });
+  } catch (error) {
+    res.status(500).json({ 
+      status: 'ERROR', 
+      timestamp: new Date().toISOString(),
+      error: error.message 
+    });
+  }
+});
+
+// Database status endpoint
+app.get('/db-status', async (req, res) => {
+  try {
+    const dbInitializer = require('./database/init');
+    const dbUtils = require('./database/utils');
+    
+    const status = await dbInitializer.getStatus();
+    const validation = await dbUtils.validateSchema();
+    
+    res.json({
+      timestamp: new Date().toISOString(),
+      ...status,
+      schema: validation
+    });
+  } catch (error) {
+    res.status(500).json({ 
+      error: error.message,
+      timestamp: new Date().toISOString()
+    });
+  }
+});
+
+// API routes will be added here
+// app.use('/api/auth', require('./routes/auth'));
+// app.use('/api/user', require('./routes/user'));
+// app.use('/api/bookmarks', require('./routes/bookmarks'));
+
+// Error handling middleware
+app.use((err, req, res, next) => {
+  console.error(err.stack);
+  res.status(500).json({ 
+    error: 'Something went wrong!',
+    timestamp: new Date().toISOString()
+  });
+});
+
+// 404 handler
+app.use((req, res) => {
+  res.status(404).json({ 
+    error: 'Route not found',
+    timestamp: new Date().toISOString()
+  });
+});
+
+module.exports = app;