feat(core): setup modular NestJS backend with JWT auth and MongoDB

### Summary
Initialized a scalable NestJS backend with modular architecture, JWT authentication, and MongoDB integration — forming a production-ready foundation for contests and alerts.

### Added
- Modular structure for Auth, Users, Contests, and Alerts
- MongoDB integration via Mongoose and env management
- JWT-based authentication with Passport strategy and guards
- Full user lifecycle: signup, signin, signout, token refresh
- bcrypt password hashing (12 salt rounds)
- Zod validation with global validation pipes
- Protected routes and user preference management
- Documentation setup (AUTH.md, USERS.md)

### Infrastructure
- Jest testing: 56 test cases across 12 suites
- ESLint + Prettier for code quality
- Environment-based configuration management
- Production-ready error handling and security

### API Endpoints
- `POST /auth/signup` — Register new user
- `POST /auth/signin` — Login user
- `POST /auth/signout` — Logout user (protected)
- `POST /auth/refresh` — Refresh JWT token
- `GET /users/profile` — Get user profile (protected)
- `PUT /users/profile` — Update user profile (protected)

Author: Celestial-0

README.md

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+# ==============================================
+# CodeNotify - Environment Configuration
+# ==============================================
+# Copy this file to .env.local and fill in the values
+
+# ==============================================
+# Server Configuration
+# ==============================================
+PORT=3000
+NODE_ENV=development
+
+# ==============================================
+# Database Configuration
+# ==============================================
+# For local development (default MongoDB port)
+MONGO_URI=mongodb://localhost:27017/codenotify
+
+# For MongoDB Atlas (cloud)
+# MONGO_URI=mongodb+srv://<username>:<password>@<cluster>.mongodb.net/codenotify?retryWrites=true&w=majority
+
+# ==============================================
+# Authentication
+# ==============================================
+# Generate a strong secret key (at least 10 characters)
+# You can use: node -e "console.log(require('crypto').randomBytes(32).toString('hex'))"
+JWT_SECRET=your_jwt_secret_key_here_change_this_in_production
+JWT_REFRESH_SECRET=your_jwt_refresh_secret_key_here_change_this_in_production
+
+# ==============================================
+# Contest Platform APIs
+# ==============================================
+# Codeforces API endpoint (optional, can be left empty for now)
+CODEFORCES_API=https://codeforces.com/api
+
+# LeetCode API endpoint (optional, can be left empty for now)
+LEETCODE_API=https://leetcode.com/graphql
+
+# ==============================================
+# WhatsApp Cloud API (Meta Business)
+# ==============================================
+# Get these from: https://developers.facebook.com/apps
+# Optional - only needed when implementing WhatsApp notifications
+
+# Your WhatsApp Business API Access Token
+WHATSAPP_API_KEY=your_whatsapp_api_access_token
+
+# Your WhatsApp Phone Number ID
+WHATSAPP_PHONE_ID=your_whatsapp_phone_id
+
+# Your WhatsApp Business Account ID
+WHATSAPP_BUSINESS_ACCOUNT_ID=your_business_account_id

backend/.env.example

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+{
+  "singleQuote": true,
+  "trailingComma": "all"
+}