Protect against frontrunning and sandwich attacks on subscription charges

[Smartdevs17]

Description

Subscription charges are visible in mempool and vulnerable to MEV. Implement protection mechanisms.

Acceptance Criteria

• Commit-reveal scheme for large charges
• Slippage protection with max fee bounds
• Private mempool integration option
• MEV extraction monitoring
• Gas price analysis for attack detection
• Documentation of MEV threat model

Technical Scope

• contracts/subscription/src/lib.rs
• Edge: commit-reveal UX impact, private mempool censorship, gas bidding analysis