Flutter Realtime Chat & Notifications Upgrade

We need an expert to audit and improve:

* Incoming call notification reliability
* FCM/APNs delivery (foreground, background, killed state)
* Agora call connection and reconnection handling
* Call lifecycle tracking and analytics
* Wallet deduction and session management
* Realtime state synchronization

Goal: Achieve production-grade reliability similar to leading calling/consultation apps.

Only apply if you have hands-on experience with Flutter, Agora, Firebase FCM, CallKit/full-screen call notifications, and large-scale realtime systems.

We have developed but sometime problems is coming in production (live)