Slice A3c: Enable PropertyNameCaseInsensitive for FastEndpoints JSON deserialization

- Set PropertyNameCaseInsensitive = true in AddFastEndpoints config
- Enables flexible JSON property name handling (PascalCase/camelCase)
- Resolves validation issues with API request deserialization

Source: AGENTS.md Blockers Must Be Actionable
Decision: Simplify JSON config to PropertyNameCaseInsensitive only

Test Result: Gate 3 API Test PASSED ✅
- HTTP 202 Accepted response
- Shadow run job queued (ID: 2546f1f9-9e24-4c28-9ca2-7425af27ceac)
- Hangfire job tracking enabled

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

src/KArtSell.Host/Program.cs

@@ -161,7 +161,10 @@ builder.Services.AddAuthorization();
 builder.Services.AddSignalR();
 builder.Services.AddSignalEngineModule();
 builder.Services.AddModelOperationsModule();
-builder.Services.AddFastEndpoints();  // AFTER modules registered (so their endpoints are included)
+builder.Services.AddFastEndpoints(config =>
+{
+    config.SerializerOptions.PropertyNameCaseInsensitive = true;
+});  // AFTER modules registered (so their endpoints are included)
 builder.Services.AddEndpointsApiExplorer();
 builder.Services.AddSwaggerGen();