Private 10K-User Mobile Chat

I’m building a secure, WhatsApp-style chat application that runs exclusively on mobile. It must serve up to 10,000 registered users smoothly on both iOS and Android, delivering:

• Real-time one-to-one and group messaging
• High-quality voice and video calls
• Quick, reliable file sharing (images, docs, short clips)

I expect clean, well-documented source code, a lightweight back-end that scales horizontally, and solid encryption of data in transit and at rest. Push notifications should be instant, and the UI should feel native on each platform.

Please factor in basic user management (registration, profile photo, status), graceful handling of intermittent connectivity, and straightforward deployment instructions so I can spin up new environments without hassle.

Acceptance criteria
1. A TestFlight/iOS build and an Android APK demonstrating all three core features.
2. Back-end deployed to my cloud account, load-tested for 10,000 concurrent connections.
3. Code repository with README covering setup, environment variables, and build steps.

If you have experience with tech such as WebSocket, gRPC, Firebase, Socket.IO, Erlang/Elixir, Swift, Kotlin, React Native, or Flutter, let me know—happy to choose the stack together as long as it meets the performance target.