Write the Event Indexing Guide

[codebestia]

Document how to build a reliable off-chain indexer over Shade's events — the foundation for dashboards, notifications, reporting, and reconciliation — including how to handle reorgs, gaps, and backfills.

Proposed steps:

• Create docs/guides/indexing-events.md.
• Document the data sources: RPC getEvents, the pagination/cursor model, and retention limits that force periodic backfills from an archive.
• Provide a recommended relational schema for merchants, invoices, payments, refunds, subscriptions, and tickets derived from the event stream.
• Document the ingestion loop: cursor persistence, batching, idempotent upserts keyed by ledger sequence and event index, and gap detection.
• Document reconciliation: periodically comparing indexed state against on-chain reads (get_invoice, get_merchant_analytics) and how to repair drift.
• Document reorg and failed-transaction handling.
• Include a worked minimal indexer example for one event type.

Acceptance criteria:

• docs/guides/indexing-events.md exists.
• A concrete schema and ingestion loop are specified.
• Idempotency, gap detection, and backfill strategies are documented.
• A reconciliation procedure against on-chain state is included.