feat(api): block-playout history + sequential-schedule validation endpoints

Adds the REST surface for the #145 troubleshooting leftovers (#158 items 4-5):

- GET /api/playouts/{id}/blocks - blocks a block playout schedules
- GET /api/playouts/{id}/blocks/{blockId}/history - paged block history
- GET /api/playouts/history/{id} - decode a history row by id
- POST /api/troubleshoot/validate-schedule - validate sequential YAML

New MediatR queries GetPlayoutHistoryDetails (Either, 404 unknown row /
422 malformed JSON) and ValidateSequentialSchedule (wraps
ISequentialScheduleValidator, never throws). DecodePlayoutHistoryHandler
and the new by-id handler now share PlayoutHistoryDecoder. Regenerated
openapi/v1.json.

Refs #145 #158

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

ErsatzTV/Controllers/Api/TroubleshootController.cs

@@ -1,3 +1,4 @@
+using System.ComponentModel.DataAnnotations;
 using System.IO.Abstractions;
 using System.Text.Json;
 using System.Text.Json.Serialization;
@@ -6,6 +7,7 @@ using ErsatzTV.Application;
 using ErsatzTV.Application.MediaItems;
 using ErsatzTV.Application.Troubleshooting;
 using ErsatzTV.Application.Troubleshooting.Queries;
+using ErsatzTV.Controllers.Api.Requests;
 using ErsatzTV.Core;
 using ErsatzTV.Core.Api.Troubleshooting;
 using ErsatzTV.Core.Domain;
@@ -71,6 +73,36 @@ public class TroubleshootController(
             info.VideoToolboxCapabilities);
     }
 
+    [HttpPost("api/troubleshoot/validate-schedule", Name = "ValidateSequentialSchedule")]
+    [Tags("Troubleshooting")]
+    [EndpointSummary("Validate a sequential schedule YAML document")]
+    [EndpointDescription(
+        "Validates a sequential-schedule YAML string against the full (or import) schema. Returns whether it is " +
+        "valid, any validation messages, and the JSON conversion of the YAML. Parse/validator errors are reported " +
+        "as messages (IsValid=false), never as a 500.")]
+    [EndpointGroupName("general")]
+    [ProducesResponseType(typeof(ValidateSequentialScheduleResponseModel), StatusCodes.Status200OK)]
+    [ProducesResponseType(typeof(ProblemDetails), StatusCodes.Status400BadRequest)]
+    public async Task<IActionResult> ValidateSchedule(
+        [Required] [FromBody] ValidateSequentialScheduleRequest request,
+        CancellationToken cancellationToken)
+    {
+        if (string.IsNullOrWhiteSpace(request.Yaml))
+        {
+            return new BadRequestObjectResult(
+                new ProblemDetails
+                {
+                    Status = StatusCodes.Status400BadRequest,
+                    Title = "Validation failed",
+                    Detail = "[Yaml] must not be empty"
+                });
+        }
+
+        ValidateSequentialScheduleViewModel result = await mediator.Send(request.ToQuery(), cancellationToken);
+        return new OkObjectResult(
+            new ValidateSequentialScheduleResponseModel(result.IsValid, result.Messages, result.Json));
+    }
+
     [HttpHead("api/troubleshoot/playback.m3u8")]
     [HttpGet("api/troubleshoot/playback.m3u8")]
     [Tags("Troubleshooting")]