feat(api): REST endpoints for templates, template groups, items + block copy

Adds TemplateController mirroring BlockController: groups CRUD, template
CRUD, item replace (PUT full-replace via ReplaceTemplateItems), and a
copy endpoint. Also wires the missing POST /api/blocks/{id}/copy since
CopyBlock's CQRS shape is directly analogous to CopyTemplate.

Fixes CreateTemplateHandler to validate the target template group exists
before insert (previously an unhandled FK violation -> 500), matching the
CreateBlockHandler fix (1e34e00e) for the same class of bug.

New DTOs in ErsatzTV.Core/Api/Scheduling; TimeSpan start times serialize
the same way as PlayoutResponseModel.DailyRebuildTime.

#144 S2 (#162)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

ErsatzTV.Core/Api/Scheduling/TemplateGroupResponseModel.cs

@@ -0,0 +1,4 @@
+#nullable enable
+namespace ErsatzTV.Core.Api.Scheduling;
+
+public record TemplateGroupResponseModel(int Id, string Name, int TemplateCount);

ErsatzTV.Core/Api/Scheduling/TemplateItemResponseModel.cs

@@ -0,0 +1,4 @@
+#nullable enable
+namespace ErsatzTV.Core.Api.Scheduling;
+
+public record TemplateItemResponseModel(int BlockId, string BlockName, int BlockMinutes, TimeSpan StartTime);

ErsatzTV.Core/Api/Scheduling/TemplateResponseModel.cs

@@ -0,0 +1,4 @@
+#nullable enable
+namespace ErsatzTV.Core.Api.Scheduling;
+
+public record TemplateResponseModel(int Id, int TemplateGroupId, string GroupName, string Name);

ErsatzTV.Core/Api/Scheduling/TemplateWithItemsResponseModel.cs

@@ -0,0 +1,9 @@
+#nullable enable
+namespace ErsatzTV.Core.Api.Scheduling;
+
+public record TemplateWithItemsResponseModel(
+    int Id,
+    int TemplateGroupId,
+    string GroupName,
+    string Name,
+    List<TemplateItemResponseModel> Items);