docs: Indexer architecture & operations guide

[joelpeace48-cell]

Background & current state

The indexer (Epic D) becomes core infrastructure, but there's no operational documentation for it (cursors, reorgs, backfill, rollups, recovery).

Goal

Author docs/INDEXER.md: architecture + operations for the event indexer.

Outline

• Architecture: sources, cursors, projections, rollups (NEW-024/026).
• Operations: monitoring (lag NEW-030), reorg handling (NEW-025), backfill (NEW-028), warehouse export (NEW-029), recovery from corruption (links DR NEW-047).

Task breakdown

• Write docs/INDEXER.md matching the implementation.
• Include runbook-style recovery steps.
• Cross-link from architecture + runbook.

Acceptance criteria

• An operator can run and recover the indexer using the doc.

Verification

• Cross-check vs implementation; review.

Difficulty: medium · Effort: M · documentation + indexer