feat(backtest): optional date-range window

- RunBacktestUseCase gains an ExecuteAsync(strategy, DateRange?, ct) overload that
  pushes the date filter to SQL via IAnomalyRepository.ListByDateRangeAsync; the
  existing no-range overload is preserved. +1 use-case test.
- BacktestForm carries optional From/To (Moscow dates) with From<=To validation and
  a ToDateRange() helper; BacktestService threads it through. Backtest page gains two
  clearable date pickers (empty = all anomalies).
- Localization (en+ru) for the backtest date fields and the settings-validation keys
  (shared resx).

src/Marathon.UI/Pages/Anomalies/Backtest.razor

@@ -46,6 +46,25 @@
                                      data-test="backtest-bankroll" />
                 </div>
 
+                <div class="m-backtest__form-field">
+                    <label class="m-backtest__form-label">@L["Backtest.Field.From"]</label>
+                    <MudDatePicker @bind-Date="_form.From"
+                                   DateFormat="yyyy-MM-dd"
+                                   Clearable="true"
+                                   Variant="Variant.Outlined"
+                                   data-test="backtest-from" />
+                </div>
+
+                <div class="m-backtest__form-field">
+                    <label class="m-backtest__form-label">@L["Backtest.Field.To"]</label>
+                    <MudDatePicker @bind-Date="_form.To"
+                                   DateFormat="yyyy-MM-dd"
+                                   Clearable="true"
+                                   Variant="Variant.Outlined"
+                                   data-test="backtest-to" />
+                    <span class="m-backtest__form-hint">@L["Backtest.Field.DateRange.Hint"]</span>
+                </div>
+
                 <div class="m-backtest__form-field">
                     <label class="m-backtest__form-label">@L["Backtest.Field.MinScore"]</label>
                     <MudNumericField T="decimal"