[Backend] — Expose campaign metadata (prize pool, duration, branding, entry fee) through the API DTOs

[Olowodarey]

Background

Issue 1 adds start_time, end_time, prize_pool, reward_distribution,
entry_fee, category, banner_url, is_finalized to the CreatorEvent
entity, but none of the response DTOs or service mapping functions expose
them yet:

• EventResponseDto (`src/creator-events/dto/event-response.

Goal

Thread the new CreatorEvent fields (and matching ContractEvent fields
from Issue 1) through to all event-facing response DTOs.

Tasks

1. EventResponseDto: add
2. EventByCodeResponseDto: add the same 6 fields. Update
3. SearchEventResultDto: add category: string,
4. EventStatsResponseDto: add prizePool: string and
   500 XLM, split 40/30/20/5/5" without a second request.
5. Reward distribution validation helper: add a small shared util (e.g.

Acceptance Criteria

• GET /creator-events/:id response includes prizePool,
• GET /creator-events/invite/:code (public) response includes the same
• GET /creator-events/search?q=...&category=football filters results
• GET /creator-events/:id/stats includes prizePool and