Create historical APY fetch method in AnalyticsModule

[Code-Paragon]

Problem

The AnalyticsModule has a pending TODO to fetch historical APY data from an off-chain indexer, which is necessary for the dashboard charts.

Expected behavior

A new asynchronous method getHistoricalAPY(timeframe: string) is added to AnalyticsModule that accepts standard timeframes (e.g., "7d", "30d") and returns an array of APY data points.

Files to update

• src/modules/analytics.ts
• docs/api-reference.md

Project relevance

This provides the data necessary for the yieldstream-dashboard to render the APY charts mentioned in the project overview.

Acceptance criteria

• Feature works as described
• Tests are added or updated where needed
• Documentation is updated where needed
• Method returns an array of objects shaped as { timestamp: number, apy: string }
• No TypeScript, lint, test, or build errors