Index Chainlink AnswerUpdated oracle events (Yearn peg monitoring L3)

[spalen0]

Context

Layer 3 (indexer side) of Yearn pegged-asset & oracle monitoring. Real-time capture of Chainlink oracle updates so no round is missed between polls. The alert consumer lives in yearn/monitoring (separate issue). Tracked by the peg-monitoring epic in yearn/monitoring.

Scope

• Add a ChainlinkAggregator contract per chain in apps/indexer/config.yaml with event:
  AnswerUpdated(int256 indexed current, uint256 indexed roundId, uint256 updatedAt).
• Add an AnswerUpdated entity to schema.graphql (aggregator/feed address, roundId, answer, updatedAt, block, tx).
• Add a handler in src/EventHandlers.ts (same shape as the existing event handlers).
• Address sourcing (important): AnswerUpdated is emitted by the underlying aggregator, which rotates on phase upgrades, and its indexed params are current / roundId — not the address — so Envio eventFilters cannot scope it by feed. Statically list the current aggregator addresses resolved from each feed proxy's aggregator(). Document the resolution step and a refresh procedure for phase upgrades (a phase change is itself a risk signal — it surfaces as staleness in L2).
• Feeds to cover: the Chainlink feeds behind the monitored peg set — cbBTC/USD, BTC/USD, USDC/USD, USDT/USD, USDS/USD, USDe/USD, LBTC/BTC, …

Acceptance criteria

• Indexer codegens and syncs; AnswerUpdated rows are queryable via GraphQL.

References

• Precedent: existing TimelockController indexing in this repo.
• Envio 3.0.1 contractRegister / wildcard docs.
• Consumer + alerting issue: in yearn/monitoring.