Implement round history query

Allow querying past round details for completed rounds.

**Acceptance Criteria:**
- Add `get_round_history(group_id) -> Vec<RoundInfo>` function
- Return all completed rounds with recipient, amounts, timestamps
- Paginate with offset and limit
- Include round completion status

**Files:** `contracts/sorosave/src/payout.rs`, `contracts/sorosave/src/lib.rs`