feat(indexer): add structured batch start/completion logs

[KingDavid9999]

Summary

• Adds structured batch-level logging to processIndexerChainEvents in src/utils/indexer-event-processor.utils.ts, the shared function that processes an incoming batch of chain events (currently only called by processTradeEvents).
• Emits an info-level log (indexer_batch_started) before processing starts, with from_ledger, to_ledger (min/max ledger across the batch), and batch_size (raw event count received, before dedup).
• Emits a debug-level log (indexer_batch_completed) after processing finishes, with from_ledger, to_ledger, events_processed (unique event count after dedup), and duration_ms (measured with the same monotonic clock helper already used for per-event timing).
• Both logs fire exactly once per batch call — the existing per-event log inside the processing loop is untouched, so per-ledger logging isn't duplicated.
• Updated docs/indexer/EVENT_PROCESSING.md with a new section documenting the two log shapes and field meanings.

Note on batch_size semantics: this server only receives events: ChainEvent[] — it has no visibility into the ledger range the external polling worker actually requested (see PIPELINE_REFERENCE.md). So from_ledger/to_ledger/batch_size are derived from the ledgers present on the events in the batch, not from an externally-supplied range. Flagging this in case the intended semantics were "ledger count in the requested range" rather than "event count in this call" — happy to add explicit range params if so.

Testing

• pnpm lint
• pnpm build
• pnpm exec prisma generate — no schema changes
• pnpm test -- indexer-event-processor.utils.test.ts — 8/8 passing, including 4 new tests for batch-start fields, batch-completion fields, single-log-per-batch, and the no-ledger edge case
• pnpm test -- indexer-pipeline.integration.test.ts — unaffected, 3/3 passing

Checklist

• Linked issue or backlog item (Add structured log for each indexer batch start and completion with ledger range and event count #648)
• No secrets or live credentials added
• Docs updated (EVENT_PROCESSING.md)
• Change is scoped to one problem (batch start/completion logging only)

Closes #648

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