Improve backtest lazy loading and test automation

docs/roadmap/backlog/DATA-DB-007-gc-databento-contract-mapping.yaml

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+id: DATA-DB-007
+title: Databento GC Contract Mapping for Backtests
+status: backlog
+priority: P1
+effort: M
+depends_on:
+  - DATA-DB-001
+tags:
+  - databento
+  - futures
+  - backtests
+summary: Add real Databento futures contract mapping for GC backtests so the page can support gold futures without fail-closed restrictions.
+acceptance_criteria:
+  - Backtest-page Databento runs support GC without requiring users to know raw contract symbols.
+  - Contract selection or front-month rollover rules are explicit and test-covered.
+  - The selected contract path yields non-empty historical price data for supported windows.
+  - Browser validation confirms the GC path works from `/{workspace_id}/backtests` with no visible runtime error.
+technical_notes:
+  - Current hardening work intentionally fail-closes GC on the backtest page because the raw `GC` symbol does not resolve reliably in Databento historical requests.
+  - Follow-up work should decide between explicit contract selection, continuous mapping, or deterministic rollover logic before re-enabling GC in the Databento path.