*Core Features 1. Real-Time Messaging One-to-one real-time chat Built using Node.js + Socket.io (or equivalent real-time engine) Messages should be delivered instantly when the receiver is online 2. Offline Message Delivery If the receiver’s mobile is offline / switched off / no internet, messages must be: Stored securely Automatically delivered when the user comes online (like WhatsApp) 3. Media & Attachments Send and receive: Text messages Images PDF files Chat window should: Auto-download attachments Show inline preview for images and PDFs (similar to WhatsApp) 4. Backend Message Sending (PHP API) Ability to send text and PDF messages from backend PHP-based API for: Sending messages Sending attachments Useful for system notifications or admin-triggered messages 5. Contact List & Chat UI Contact list should display: Contact name (if saved) Otherwise, mobile number (like WhatsApp) Show: Unread message count per contact New message indicators Read / unread status handling 6. Notifications Push notifications for new messages Notification count per contact and globally Works reliably when app is in background 7. Message Storage Strategy Mobile app must: Store previous messages, images, and PDFs locally Old messages do not need to remain on the server once delivered and synced Server storage should be minimal and optimized *Technical Requirements Real-time engine: Node.js + Socket.io Backend API: PHP (for sending messages from server/admin panel) Database: Developer recommendation (MySQL / MongoDB / etc.) Mobile App: Android (iOS optional – please specify) Security: Proper authentication & secure file handling *Deployment & Delivery Developer must: Install and configure backend on my VPS Deploy and test real-time server Deliver complete mobile application (APK / source code) Provide basic documentation for setup and APIs * Preferred Developer Profile Proven experience in: Real-time chat systems Socket.io / WebSockets Mobile app development Clean, scalable, and well-documented code