Clean up per-validator vote data on swap completion

[paulee]

Problem

During a security audit of the ink! smart contract, I found that per-validator vote entries are never cleaned up, creating dead storage that grows indefinitely.

What happens:

1. When validators vote on swaps, entries are written to swap_confirm_votes[(swap_id, validator)] and swap_timeout_votes[(swap_id, validator)]
2. When a swap completes or times out, only the aggregate swap_confirm_vote_count and swap_timeout_vote_count are removed (lines 758-759, 840-841)
3. The individual per-validator boolean entries are never deleted — they accumulate forever
4. Same issue in the request voting system: request_votes[(request_id, validator)] entries are never removed by clear_request_data

Over time this creates unbounded dead storage growth proportional to num_swaps × num_validators.

Fix

Track voters per swap/request using Vec<AccountId> mappings, then iterate and remove all per-validator entries on completion:

• Added swap_confirm_voters, swap_timeout_voters, request_voters storage fields (Mapping<u64, Vec<AccountId>>)
• Vote recording now appends caller to the voters list
• New clean_swap_votes helper removes all per-validator entries for both confirm and timeout votes
• Updated clear_request_data to remove per-validator request_votes entries
• Both vote_confirm and timeout_swap now call clean_swap_votes at quorum instead of only removing vote counts

Test plan

• cargo test — test_timeout_cleans_up_vote_data confirms per-validator entries are removed after timeout
• Verify on testnet that vote data is properly cleaned after swap completion