feat: implement batch bet placement in a single transaction

[greatest0fallt1me]

Description:

Allow users to place multiple bets (different events or outcomes) in a single transaction to save gas and improve UX.

Requirements and Context:

• Must be secure, tested, and documented
• Should add place_bets(bets: Vec<(event_id, outcome, amount)>) (or equivalent)
• Should validate each bet and either apply all or none (atomic)
• Should emit events per bet or batch event
• Must enforce same rules as single place_bet (limits, deadline, balance)
• Should be gas-efficient (avoid redundant storage reads where possible)

Suggested Execution:

1. Fork the repo and create a branch

   git checkout -b feature/batch-bet-placement

2. Implement changes

   • Write contract: src/lib.rs (add place_bets)
   • Update validation and storage
   • Add documentation and NatSpec-style comments

3. Test and commit

   • Run tests; cover full success, partial failure (atomic revert), limits
   • Include test output and security notes

Example Commit Message:

feat: implement batch bet placement in a single transaction

Guidelines:

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