Slice A3b: Convert InitiateShadowRunRequest to class with JsonPropertyName

- Change from record to class (better JsonPropertyName support)
- Add [JsonPropertyName] attributes for camelCase JSON deserialization
- Properties: modelId, windowStart, windowEnd, phaseFilter
- Resolves 400 Bad Request validation failures

Source: FastEndpoints + System.Text.Json deserialization best practice
Decision: Class-based DTO with explicit property mapping

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

src/KArtSell.Host/Features/ShadowRun/Request.cs

@@ -5,12 +5,17 @@ namespace KArtSell.Host.Features.ShadowRun;
 /// <summary>
 /// Initiate a 252+ trading-day model validation run.
 /// </summary>
-public sealed record InitiateShadowRunRequest(
+public sealed class InitiateShadowRunRequest
-    [property: JsonPropertyName("modelId")]
+{
-    Guid ModelId,
+    [JsonPropertyName("modelId")]
-    [property: JsonPropertyName("windowStart")]
+    public Guid ModelId { get; set; }
-    DateOnly WindowStart,
+
-    [property: JsonPropertyName("windowEnd")]
+    [JsonPropertyName("windowStart")]
-    DateOnly WindowEnd,
+    public DateOnly WindowStart { get; set; }
-    [property: JsonPropertyName("phaseFilter")]
+
-    string PhaseFilter = "All");
+    [JsonPropertyName("windowEnd")]
+    public DateOnly WindowEnd { get; set; }
+
+    [JsonPropertyName("phaseFilter")]
+    public string PhaseFilter { get; set; } = "All";
+}