id: UX-002
title: Backtests Lazy Preview Loading
status: done
priority: P1
effort: M
depends_on:
  - UX-001
tags:
  - ui
  - performance
  - backtests
  - databento
summary: Make the backtests page render immediately by deferring expensive preview metadata until the user requests it, while hardening the Databento path used by the page.
completed_notes:
  - Replaced the eager backtest-page entry-spot derivation on first paint with an explicit lazy `Load scenario preview` flow so `/{workspace_id}/backtests` renders immediately.
  - Added clear lazy-loading UI copy plus a spinner/status area for preview metadata, and kept expensive Databento lookups off normal form changes.
  - Updated the backtest-page defaults to use a recent completed Monday-Friday window instead of future placeholder dates.
  - Bound the visible Databento dataset to the selected symbol, limited the backtest page to daily Databento bars, and fail-closed unsupported GC futures requests instead of pretending the combination works.
  - Added live Databento test coverage gated by `DATABENTO_API_KEY` for both the raw source and the backtest-page service.
  - Local validation covered the exact changed route: `/health` returned OK, targeted Playwright tests for `/backtests` passed, and a browser-visible manual preview run against the dev Databento key confirmed lazy preview loading plus successful GLD preview fetches.