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)
9a3b835c95