Add reentrancy protection for subscription charging contract

[Smartdevs17]

Description

The subscription charging function may be vulnerable to reentrancy. Implement comprehensive reentrancy protection.

Acceptance Criteria

• Reentrancy guard on charge function
• Checks-effects-interactions pattern verified
• Cross-contract reentrancy protection
• Formal verification of reentrancy safety
• Fuzz testing for reentrancy scenarios
• Documentation of threat model

Technical Scope

• contracts/subscription/src/lib.rs
• Edge: read-only reentrancy, cross-function reentrancy, flash loan attack surface