feat: add recurring group auto restart

[TUPM96]

Closes #16
/claim #16

Summary

• add recurring group creation with an auto_restart flag stored on group state
• let active recurring group members opt out of the next cycle
• restart completed recurring groups at round 1 with opted-out members removed and payout order rotated for the new cycle
• keep non-recurring groups on the existing completed lifecycle

Validation

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

Payout details can be provided privately on acceptance.

[Copilot]

Copilot encountered an error and was unable to review this pull request. You can try again by re-requesting a review.