[Backend] — `EventFinalized` indexer handler + prize payout tracking

[Olowodarey]

Background

The contract's new finalize_event (contract Issue 6) is permissionless,
ranks participants via the leaderboard, splits prize_pool according to
reward_distribution, and emits an event with the final
leaderboard + per-user payout amounts.

Goal

Index the EventFinalized contract event: mark the CreatorEvent as
finalized, persist the final ranked leaderboard
(CreatorEventLeaderboardEntry, from Issue 4) with payout amounts, and
expose a payouts API so the frontend can show "you won X XLM, claim it".

Tasks

1. detectEventType() (line 297): add
2. extractEventData(): new case —
3. processEventByType() (switch, ~line 480): add
4. New entity src/matches/entities/creator-event-payout.entity.ts:
5. Migration: <timestamp>-CreateCreatorEventPayout.ts.
6. handleEventFinalized(data):
   • Look up the CreatorEvent by on_chain_event_id = data.event_id; warn
   • If event.is_finalized already (idempotency — events can be

Acceptance Criteria

• detectEventType recognizes EventFinalized topics
• handleEventFinalized() creates one CreatorEventLeaderboardEntry and
• Re-processing the same EventFinalized event (idempotency / replay)
• GET /creator-events/:id/payouts returns all payouts for a finalized
• GET /creator-events/:id/payouts/:address returns 404 for an address