Improve backtest lazy loading and test automation

docs/roadmap/done/UX-002-backtests-lazy-preview-loading.yaml

@@ -0,0 +1,20 @@
+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.