fix(api): validate date-range month/day bounds on playout PUT actions

AlternateScheduleSelector.GetScheduleForDate constructs
new DateTime(year, StartMonth, StartDay) and only recovers from an
out-of-range *day* (rolling to the 1st of the next month); an out-of-range
*month* (e.g. 0 or 13) throws again from inside that recovery path and is
never caught, crashing playout building.

Validate StartMonth/EndMonth (1..12) and StartDay/EndDay (1..31) on both
PUT /api/playouts/{id}/alternate-schedules and PUT /api/playouts/{id}/templates
whenever an item sets LimitToDateRange, returning 422 before dispatching to
the mediator. Both actions already documented 422 in their ProducesResponseType
metadata.

ErsatzTV/Controllers/Api/PlayoutController.cs

@@ -306,6 +306,11 @@ public class PlayoutController(IMediator mediator) : ControllerBase
             return BaseError.New("[Items] must contain at least one alternate schedule").ToErrorResult();
         }
 
+        foreach (BaseError error in ValidateDateRanges(items.Select(ToDateRangeCheck)))
+        {
+            return error.ToErrorResult();
+        }
+
         List<ProgramScheduleViewModel> schedules =
             await mediator.Send(new GetAllProgramSchedules(), cancellationToken);
         var scheduleIds = schedules.Select(s => s.Id).ToHashSet();
@@ -389,6 +394,11 @@ public class PlayoutController(IMediator mediator) : ControllerBase
 
         List<PlayoutTemplateItemRequest> items = request.Items ?? [];
 
+        foreach (BaseError error in ValidateDateRanges(items.Select(ToDateRangeCheck)))
+        {
+            return error.ToErrorResult();
+        }
+
         List<TemplateViewModel> templates = await mediator.Send(new GetAllTemplates(), cancellationToken);
         var templateIds = templates.Select(t => t.Id).ToHashSet();
         var missingTemplateIds = items.Select(i => i.TemplateId).Distinct()
@@ -444,6 +454,53 @@ public class PlayoutController(IMediator mediator) : ControllerBase
         return result.ToDeletedResult();
     }
 
+    // Guards against AlternateScheduleSelector crashing on out-of-range dates: it constructs
+    // `new DateTime(year, StartMonth, StartDay)` and only recovers from an overflowing *day*
+    // (rolling to the 1st of the next month); an out-of-range *month* throws again from inside
+    // that recovery path and is never caught. Only checked when LimitToDateRange is set, since
+    // the fields are ignored otherwise.
+    private static (bool LimitToDateRange, int StartMonth, int StartDay, int EndMonth, int EndDay) ToDateRangeCheck(
+        PlayoutAlternateScheduleItemRequest item) =>
+        (item.LimitToDateRange, item.StartMonth, item.StartDay, item.EndMonth, item.EndDay);
+
+    private static (bool LimitToDateRange, int StartMonth, int StartDay, int EndMonth, int EndDay) ToDateRangeCheck(
+        PlayoutTemplateItemRequest item) =>
+        (item.LimitToDateRange, item.StartMonth, item.StartDay, item.EndMonth, item.EndDay);
+
+    private static Option<BaseError> ValidateDateRanges(
+        IEnumerable<(bool LimitToDateRange, int StartMonth, int StartDay, int EndMonth, int EndDay)> items)
+    {
+        foreach ((bool limitToDateRange, int startMonth, int startDay, int endMonth, int endDay) in items)
+        {
+            if (!limitToDateRange)
+            {
+                continue;
+            }
+
+            if (startMonth is < 1 or > 12)
+            {
+                return Some(BaseError.New($"[StartMonth] {startMonth} must be between 1 and 12"));
+            }
+
+            if (endMonth is < 1 or > 12)
+            {
+                return Some(BaseError.New($"[EndMonth] {endMonth} must be between 1 and 12"));
+            }
+
+            if (startDay is < 1 or > 31)
+            {
+                return Some(BaseError.New($"[StartDay] {startDay} must be between 1 and 31"));
+            }
+
+            if (endDay is < 1 or > 31)
+            {
+                return Some(BaseError.New($"[EndDay] {endDay} must be between 1 and 31"));
+            }
+        }
+
+        return Option<BaseError>.None;
+    }
+
     private static PlayoutResponseModel ToResponse(PlayoutNameViewModel vm) =>
         PlayoutResponseModel.From(
             vm.PlayoutId,