feat: add backtesting functionality with UI and API endpoints

- Introduced backtesting page and fragment in the dashboard for running backtests and viewing recent reports.
- Implemented backtest run logic with configuration options including event path, starting balances, trade capital, and fee profiles.
- Added recent backtest reports storage and retrieval.
- Created a new strategy module for statistical arbitrage experiments with validation on configuration parameters.
- Updated settings to include parameters for the statistical arbitrage strategy.
- Enhanced dashboard controls to support the new strategy mode.
- Added unit tests for backtesting functionality and strategy validation.
- Updated templates for backtesting UI integration.

web/templates/partials/controls.html

@@ -131,6 +131,12 @@
             <option value="paper" {{ sel }}>paper</option>
             {% set sel = "selected" if strategy_mode == "live" else "" %}
             <option value="live" {{ sel }}>live</option>
+            {% if strategy_stat_arb_enabled %} {% set sel = "selected" if
+            strategy_mode == "stat_arb_experiment" else "" %}
+            <option value="stat_arb_experiment" {{ sel }}>
+              stat_arb_experiment
+            </option>
+            {% endif %}
           </select>
         </label>
         <label class="field">