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/ListingTests.cs

@@ -97,7 +97,7 @@ public class ListingTests : ServiceTestFixture
         var agent = new NovelAgentService(
             Db.Context,
             new ScriptedModelClient([[new AgentTextBlock("Reply.")]]),
-            new NovelAgentToolset(Projects, Characters, Arcs, Chapters, Beats, Scenes, Tags, Questions),
+            new NovelAgentToolset(Projects, Characters, Arcs, Chapters, Beats, Scenes, Tags, Questions, NullLogger<NovelAgentToolset>.Instance),
             Options.Create(new AgentOptions()),
             NullLogger<NovelAgentService>.Instance);