feat(DATA-DB-004): add Databento settings UI and independent scenario config

- Updated backtests page with Data Source card
  - Data source selector (databento/yfinance/synthetic)
  - Dataset dropdown (XNAS.BASIC, GLBX.MDP3)
  - Resolution dropdown (ohlcv-1d, ohlcv-1h)
  - Cost estimate display (placeholder for now)

- Added Scenario Configuration card
  - Underlying symbol selector (GLD/GC/XAU)
  - Start/end date inputs
  - Start price input (0 = auto-derive)
  - Underlying units, loan amount, margin call LTV

- BacktestPageService updates:
  - get_historical_prices() with data_source parameter
  - get_cost_estimate() for Databento cost estimation
  - get_cache_stats() for cache status display
  - Support for injected custom provider identity
  - DataSourceInfo for provider metadata

- BacktestSettingsRepository integration:
  - Load/save settings per workspace
  - Default values from BacktestSettings.create_default()

- Test update: TLT validation message changed to reflect
  new multi-symbol support (GLD, GC, XAU)

tests/test_backtest_ui.py

@@ -151,7 +151,7 @@ def test_backtest_page_service_keeps_fixture_history_while_using_caller_portfoli
                 "loan_amount": 68000.0,
                 "margin_call_ltv": 0.75,
             },
-            "BT-001A backtests are currently limited to GLD on this page",
+            "Backtests support symbols: GLD, GC, XAU",
         ),
         (
             {