Add Serilog console logging across the API

Information at endpoint and service-method boundaries, Debug in deeper
helpers, Warning before expected/recoverable failures (not-found,
validation, agent tool errors), Error on caught exceptions. Serilog
wraps the exception handler so request-completion logs report the
resolved status code rather than the raw exception. Never logs prose
bodies or the Anthropic API key.

src/Novelly.AppHost/AppHost.cs

@@ -3,8 +3,7 @@ var builder = DistributedApplication.CreateBuilder(args);
 // Port 5080 is pinned to match src/Novelly.Web's Vite proxy default and the curl-based
 // smoke checks in CLAUDE.md, so the API sits at the same address whether it is started
 // on its own with `dotnet run` or through this AppHost.
-var api = builder.AddProject<Projects.Novelly_Api>("api")
-    .WithHttpEndpoint(port: 5080, name: "http");
+var api = builder.AddProject<Projects.Novelly_Api>("api").WithHttpEndpoint(port: 5080, name: "http");
 
 builder.AddViteApp("web", "../Novelly.Web", "dev")
     .WithReference(api)