Initial implementation of Website Change Detection Monitor MVP

Features implemented:
- Backend API with Express + TypeScript
- User authentication (register/login with JWT)
- Monitor CRUD operations with plan-based limits
- Automated change detection engine
- Email alert system
- Frontend with Next.js + TypeScript
- Dashboard with monitor management
- Login/register pages
- Monitor history viewer
- PostgreSQL database schema
- Docker setup for local development

Technical stack:
- Backend: Express, TypeScript, PostgreSQL, Redis (ready)
- Frontend: Next.js 14, React Query, Tailwind CSS
- Database: PostgreSQL with migrations
- Services: Page fetching, diff detection, email alerts

Documentation:
- README with full setup instructions
- SETUP guide for quick start
- PROJECT_STATUS with current capabilities
- Complete technical specifications

Ready for local testing and feature expansion.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

backend/src/services/fetcher.ts

@@ -0,0 +1,128 @@
+import axios, { AxiosResponse } from 'axios';
+import * as cheerio from 'cheerio';
+import crypto from 'crypto';
+
+export interface FetchResult {
+  html: string;
+  text: string;
+  hash: string;
+  status: number;
+  responseTime: number;
+  error?: string;
+}
+
+export async function fetchPage(
+  url: string,
+  elementSelector?: string
+): Promise<FetchResult> {
+  const startTime = Date.now();
+
+  try {
+    // Validate URL
+    new URL(url);
+
+    const response: AxiosResponse = await axios.get(url, {
+      timeout: 30000,
+      maxRedirects: 5,
+      headers: {
+        'User-Agent':
+          'Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/120.0.0.0 Safari/537.36',
+        Accept:
+          'text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8',
+        'Accept-Language': 'en-US,en;q=0.5',
+        'Accept-Encoding': 'gzip, deflate, br',
+        Connection: 'keep-alive',
+        'Upgrade-Insecure-Requests': '1',
+      },
+      validateStatus: (status) => status < 500,
+    });
+
+    const responseTime = Date.now() - startTime;
+    let html = response.data;
+
+    // If element selector is provided, extract only that element
+    if (elementSelector) {
+      const $ = cheerio.load(html);
+      const element = $(elementSelector);
+
+      if (element.length === 0) {
+        throw new Error(`Element not found: ${elementSelector}`);
+      }
+
+      html = element.html() || '';
+    }
+
+    // Extract text content
+    const $ = cheerio.load(html);
+    const text = $.text().trim();
+
+    // Generate hash
+    const hash = crypto.createHash('sha256').update(html).digest('hex');
+
+    return {
+      html,
+      text,
+      hash,
+      status: response.status,
+      responseTime,
+    };
+  } catch (error: any) {
+    const responseTime = Date.now() - startTime;
+
+    if (error.response) {
+      return {
+        html: '',
+        text: '',
+        hash: '',
+        status: error.response.status,
+        responseTime,
+        error: `HTTP ${error.response.status}: ${error.response.statusText}`,
+      };
+    }
+
+    if (error.code === 'ENOTFOUND') {
+      return {
+        html: '',
+        text: '',
+        hash: '',
+        status: 0,
+        responseTime,
+        error: 'Domain not found',
+      };
+    }
+
+    if (error.code === 'ETIMEDOUT' || error.code === 'ECONNABORTED') {
+      return {
+        html: '',
+        text: '',
+        hash: '',
+        status: 0,
+        responseTime,
+        error: 'Request timeout',
+      };
+    }
+
+    return {
+      html: '',
+      text: '',
+      hash: '',
+      status: 0,
+      responseTime,
+      error: error.message || 'Unknown error',
+    };
+  }
+}
+
+export function extractTextFromHtml(html: string): string {
+  const $ = cheerio.load(html);
+
+  // Remove script and style elements
+  $('script').remove();
+  $('style').remove();
+
+  return $.text().trim();
+}
+
+export function calculateHash(content: string): string {
+  return crypto.createHash('sha256').update(content).digest('hex');
+}