Slice A3a: Add JsonPropertyName to InitiateShadowRunRequest (camelCase support)

- Support camelCase JSON properties (modelId, windowStart, windowEnd, phaseFilter)
- FastEndpoints default deserializer expects exact case match
- JsonPropertyName enables API contract flexibility (camelCase per REST convention)
- Resolves 400 Bad Request when client sends camelCase payload

Source: FastEndpoints deserialization pattern, System.Text.Json convention
Decision: Add JsonPropertyName attributes to record properties

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

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

@@ -1,10 +1,16 @@
+using System.Text.Json.Serialization;
+
 namespace KArtSell.Host.Features.ShadowRun;
 
 /// <summary>
 /// Initiate a 252+ trading-day model validation run.
 /// </summary>
 public sealed record InitiateShadowRunRequest(
+    [property: JsonPropertyName("modelId")]
     Guid ModelId,
+    [property: JsonPropertyName("windowStart")]
     DateOnly WindowStart,
+    [property: JsonPropertyName("windowEnd")]
     DateOnly WindowEnd,
+    [property: JsonPropertyName("phaseFilter")]
     string PhaseFilter = "All");