feat: implement reentrancy guards and security hardening

[greatest0fallt1me]

Description:

Add reentrancy guards and other security hardening (e.g., checks-effects-interactions, overflow checks) to all entrypoints that perform external calls (e.g., token transfer).

Requirements and Context:

• Must be secure, tested, and documented
• Should use Soroban-approved pattern for reentrancy (e.g., guard flag or ledger-based lock)
• Should protect place_bet, payouts, refunds, claim, fee withdrawal
• Should document and test attack vectors (reentrant call mid-payout)
• Should ensure no overflow in arithmetic (use checked ops)
• Must not break existing behavior

Suggested Execution:

1. Fork the repo and create a branch

   git checkout -b feature/reentrancy-guards

2. Implement changes

   • Write contract: add guard to relevant functions
   • Add reentrancy_guard module: src/reentrancy_guard.rs
   • Update all entrypoints that do external calls
   • Add documentation and NatSpec-style comments

3. Test and commit

   • Run tests; add dedicated reentrancy attempt tests
   • Include test output and security notes

Example Commit Message:

feat: implement reentrancy guards and security hardening

Guidelines:

• Minimum 95 percent test coverage
• Clear documentation
• Timeframe: 72 hours