[Backend] Background indexer/worker logs carry no correlation id - only HTTP requests get one

[grantfox-oss]

Telegram (ask questions / claim the issue here first): https://t.me/+DOylgFv1jyJlNzM0

Why this matters

logger.ts:4-18 injects requestId into log lines only from the AsyncLocalStorage store set on HTTP requests. The Soroban worker and indexerService run outside any requestContext, so a poll batch and its per-event errors share no traceable id - you cannot correlate failures belonging to one poll cycle or a specific admin replay.

Acceptance criteria

• Wrap each poll/replay batch in requestContext.run({ requestId: }, ...) so all worker logs for one cycle share an id
• Include the batch/correlation id in indexer error logs and in the replay endpoint response
• Document the field in the logging section

Files to touch

• backend/src/workers/soroban-event-worker.ts
• backend/src/logger.ts
• backend/src/services/indexerService.ts

Out of scope

• Changing the log transport/format library