Implement event subscription and real-time event listener

[big14way]

Add the ability to subscribe to contract events in real-time.

Acceptance Criteria:

• Add onEvent(eventType, callback) method
• Support event types: contribution, payout, group_created, member_joined
• Poll Horizon for new events at configurable interval
• Add unsubscribe() cleanup method
• Parse event data into typed objects

Files: new file src/events.ts, src/client.ts, src/types.ts, src/index.ts

[lustsazeus-lab]

Nice scope. One thing worth locking in early: event cursor persistence (last ledger / paging token) so restarts don't replay or miss events.

If onEvent uses at-least-once semantics with idempotency hints, downstream apps stay predictable under reconnects.

[Kheshie1]

/attempt #33

Implementation complete — PR: #78

Approach:

• New SoroSaveEventListener class polling Soroban RPC getEvents at configurable intervals
• Typed events for all 4 types: contribution, payout, group_created, member_joined
• Cursor persistence for restart resilience (at-least-once semantics)
• Convenience methods on SoroSaveClient (onEvent/unsubscribeEvent)
• 21 tests with full coverage of parsing, subscriptions, and edge cases