test(api): pin mirror-offset guide behavior; clarify guide end-default wording (#102)

- Add GetChannelGuideDataHandlerTests coverage for the mirror-channel path:
  asserts programme Start/Stop shift by PlayoutOffset and that the source
  channel's seeded PlayoutItem entities are not mutated, pinning the
  WithPlayoutOffset copy fix.
- ChannelController.GetGuide Swagger description said end defaults to "now
  plus XmltvDaysToBuild"; it actually defaults to start plus that window
  (matters when start is supplied). Reworded and regenerated v1.json.

ErsatzTV.Tests/Application/Channels/GetChannelGuideDataHandlerTests.cs

@@ -205,6 +205,41 @@ public class GetChannelGuideDataHandlerTests
         (result.End - result.Start).ShouldBe(TimeSpan.FromDays(3));
     }
 
+    [Test]
+    public async Task Handle_Should_Shift_Mirror_Channel_Programmes_By_PlayoutOffset_Without_Mutating_Source_Items()
+    {
+        PlayoutItem sourceItem = MakeItem(BaseTime, BaseTime.AddHours(1), guideGroup: 1, movieTitle: "Live Show");
+        var playoutOffset = TimeSpan.FromHours(3);
+
+        await using (TvContext context = _db.CreateContext())
+        {
+            DomainChannel source = NewChannel("1", "Source", showInEpg: false);
+            source.Playouts = [MakePlayout(source, PlayoutScheduleKind.Classic, [sourceItem])];
+
+            DomainChannel mirror = NewChannel("2", "Mirror", showInEpg: true);
+            mirror.PlayoutSource = ChannelPlayoutSource.Mirror;
+            mirror.MirrorSourceChannel = source;
+            mirror.PlayoutOffset = playoutOffset;
+
+            context.Channels.AddRange(source, mirror);
+            await context.SaveChangesAsync();
+        }
+
+        ChannelGuideResponseModel result = await MakeHandler().Handle(
+            new GetChannelGuideData(BaseTime, BaseTime.AddDays(1)),
+            CancellationToken.None);
+
+        ChannelGuideProgrammeResponseModel programme = result.Channels.Single(c => c.Number == "2").Programmes.Single();
+        programme.Title.ShouldBe("Live Show");
+        programme.Start.ShouldBe(new DateTimeOffset(BaseTime.Add(playoutOffset), TimeSpan.Zero));
+        programme.Stop.ShouldBe(new DateTimeOffset(BaseTime.AddHours(1).Add(playoutOffset), TimeSpan.Zero));
+
+        // the WithPlayoutOffset copy fix: applying the mirror offset must not mutate the source playout's
+        // own (shared, AsNoTracking) PlayoutItem entities in place.
+        sourceItem.Start.ShouldBe(BaseTime);
+        sourceItem.Finish.ShouldBe(BaseTime.AddHours(1));
+    }
+
     [Test]
     public async Task Handle_Should_Return_Empty_Programmes_For_Channel_Without_Playout()
     {

ErsatzTV/Controllers/Api/ChannelController.cs

@@ -26,8 +26,8 @@ public class ChannelController(ChannelWriter<IBackgroundServiceRequest> workerCh
     [Tags("Channels")]
     [EndpointSummary("Get the JSON channel guide (EPG)")]
     [EndpointDescription(
-        "Returns per-channel programme arrays for the EPG grid. When omitted, start defaults to now " +
+        "Returns per-channel programme arrays for the EPG grid. When omitted, start defaults to now, " +
-        "and end defaults to now plus the configured XmltvDaysToBuild window.")]
+        "and end defaults to start plus the configured XmltvDaysToBuild window.")]
     [EndpointGroupName("general")]
     [ProducesResponseType(typeof(ChannelGuideResponseModel), StatusCodes.Status200OK)]
     public async Task<ChannelGuideResponseModel> GetGuide(

ErsatzTV/wwwroot/openapi/v1.json

@@ -221,7 +221,7 @@
           "Channels"
         ],
         "summary": "Get the JSON channel guide (EPG)",
-        "description": "Returns per-channel programme arrays for the EPG grid. When omitted, start defaults to now and end defaults to now plus the configured XmltvDaysToBuild window.",
+        "description": "Returns per-channel programme arrays for the EPG grid. When omitted, start defaults to now, and end defaults to start plus the configured XmltvDaysToBuild window.",
         "parameters": [
           {
             "name": "start",