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.

tests/Novelly.Api.Tests/NovelAgentServiceTests.cs

@@ -12,7 +12,7 @@ public class NovelAgentServiceTests : ServiceTestFixture
     private NovelAgentToolset _toolset = null!;
 
     protected override void OnSetUp() =>
-        _toolset = new NovelAgentToolset(Projects, Characters, Arcs, Chapters, Beats, Scenes, Tags, Questions);
+        _toolset = new NovelAgentToolset(Projects, Characters, Arcs, Chapters, Beats, Scenes, Tags, Questions, NullLogger<NovelAgentToolset>.Instance);
 
     private NovelAgentService BuildAgent(ScriptedModelClient model) => new(
         Db.Context,