[Backend] — Entry fee: grow `prize_pool` on `UserJoinedEvent`

[Olowodarey]

Background

The contract's join_event (contract Issue 7) charges event.entry_fee
(if non-zero) from the joining user and adds it to event.prize_pool. The
indexer's current UserJoinedEvent handling only tracks
participant_count:

If entry_fee > 0, the cached `creator_events.

Goal

Capture the entry fee paid (if any) in the UserJoinedEvent event and add
it to the cached prize_pool, and add a running total of fees collected for
transparency.

Tasks

1. extractEventData() 'UserJoinedEvent' case: add
2. CreatorEvent entity (Issue 1): add
3. handleUserJoinedEvent(): after incrementing
   • event.prize_pool = (BigInt(event.prize_pool) + BigInt(data.entry_fee_paid)).toString()
   • event.total_entry_fees_collected = (BigInt(event.total_entry_fees_collected) + BigInt(data.entry_fee_paid)).toString()
4. EventStatsResponseDto (Issue 5): add

Acceptance Criteria

• A UserJoinedEvent with entry_fee_paid: "10000000" (1 XLM in
• A UserJoinedEvent with no entry_fee_paid (free event) leaves
• prize_pool arithmetic uses BigInt and is correct for values