Remove NotFoundException; services return null on lookup miss

A missing record isn't exceptional — services now return null (logged
at Info) instead of throwing, and endpoints map null to 404. Agent and
import toolsets route not-found through their existing OrNotFound
result pattern rather than a caught exception.

src/Novelly.Api/Program.cs

@@ -61,7 +61,6 @@ app.UseExceptionHandler(handler => handler.Run(async context =>
 
     var (status, title) = exception switch
     {
-        NotFoundException => (StatusCodes.Status404NotFound, "Not found"),
         AgentNotConfiguredException => (StatusCodes.Status503ServiceUnavailable, "Agent unavailable"),
         ArgumentException or InvalidOperationException => (StatusCodes.Status400BadRequest, "Invalid request"),
         _ => (StatusCodes.Status500InternalServerError, "Unexpected error")