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/scripts/db-cli.js

@@ -0,0 +1,96 @@
+#!/usr/bin/env node
+
+/**
+ * Database CLI utility for managing the bookmark manager database
+ * Usage: node scripts/db-cli.js <command>
+ */
+
+require('dotenv').config();
+const dbInitializer = require('../src/database/init');
+const dbConnection = require('../src/database/connection');
+const dbUtils = require('../src/database/utils');
+
+const commands = {
+    init: 'Initialize database with migrations',
+    status: 'Show database status and diagnostics',
+    reset: 'Reset database (development only)',
+    validate: 'Validate database schema',
+    cleanup: 'Clean up expired tokens and old data',
+    diagnostics: 'Run comprehensive database diagnostics',
+    help: 'Show this help message'
+};
+
+async function runCommand(command) {
+    try {
+        switch (command) {
+            case 'init':
+                console.log('🚀 Initializing database...');
+                await dbInitializer.initialize();
+                break;
+
+            case 'status':
+                console.log('📊 Getting database status...');
+                const status = await dbInitializer.getStatus();
+                console.log(JSON.stringify(status, null, 2));
+                break;
+
+            case 'reset':
+                if (process.env.NODE_ENV === 'production') {
+                    console.error('❌ Reset is not allowed in production');
+                    process.exit(1);
+                }
+                console.log('⚠️  Resetting database...');
+                await dbInitializer.reset();
+                break;
+
+            case 'validate':
+                console.log('🔍 Validating database schema...');
+                const validation = await dbUtils.validateSchema();
+                console.log(JSON.stringify(validation, null, 2));
+                
+                if (validation.valid) {
+                    console.log('✅ Schema validation passed');
+                } else {
+                    console.log('❌ Schema validation failed');
+                    process.exit(1);
+                }
+                break;
+
+            case 'cleanup':
+                console.log('🧹 Running database cleanup...');
+                await dbUtils.cleanup();
+                break;
+
+            case 'diagnostics':
+                console.log('🔍 Running database diagnostics...');
+                const diagnostics = await dbUtils.diagnostics();
+                console.log(JSON.stringify(diagnostics, null, 2));
+                break;
+
+            case 'help':
+            default:
+                console.log('📖 Database CLI Commands:');
+                console.log('');
+                Object.entries(commands).forEach(([cmd, desc]) => {
+                    console.log(`  ${cmd.padEnd(12)} - ${desc}`);
+                });
+                console.log('');
+                console.log('Usage: node scripts/db-cli.js <command>');
+                break;
+        }
+    } catch (error) {
+        console.error('❌ Command failed:', error.message);
+        process.exit(1);
+    } finally {
+        await dbConnection.close();
+    }
+}
+
+// Get command from command line arguments
+const command = process.argv[2];
+
+if (!command) {
+    runCommand('help');
+} else {
+    runCommand(command);
+}