Fix Reports 400 errors: normalize DateTime to UTC and restore camelCase JSON

Query-bound DateTimes have Kind=Unspecified which Npgsql rejects for
timestamptz columns, causing InvalidOperationException caught as 400.
Also add explicit CamelCase naming policy lost when AddJsonOptions was
introduced for the enum converter.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/Purrse.Api/Program.cs

@@ -105,6 +105,7 @@ builder.Services.AddSignalR();
 builder.Services.AddControllers()
     .AddJsonOptions(options =>
     {
+        options.JsonSerializerOptions.PropertyNamingPolicy = System.Text.Json.JsonNamingPolicy.CamelCase;
         options.JsonSerializerOptions.Converters.Add(new System.Text.Json.Serialization.JsonStringEnumConverter());
     });
 builder.Services.AddEndpointsApiExplorer();