Add round deadline extension

[er1c-cartman]

Summary

• add admin-only extend_deadline(admin, group_id, extra_seconds) for the active round
• track deadline_extensions on each RoundInfo, resetting to zero for each new round
• reject zero/oversized extensions, non-active groups, non-admin callers, and more than two extensions per round
• emit a dl_ext event with the group, round, new deadline, and extension count

Fixes #77.

Tests

• cargo test --package sorosave
• cargo fmt --check
• cargo clippy --package sorosave --all-targets -- -D warnings
• git diff --check

Notes

This follows the issue acceptance criteria as a direct group-admin deadline extension flow: at most one cycle_length per call and at most two extensions per round.