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/AnthropicClientTests.cs

@@ -1,3 +1,4 @@
+using Microsoft.Extensions.Logging.Abstractions;
 using Microsoft.Extensions.Options;
 using Novelly.Api.Agent;
 using Novelly.Api.Common;
@@ -13,7 +14,7 @@ public class AnthropicClientTests
         // (listing conversations, reading a transcript). Throwing at construction would
         // take those down on any install that has not configured a key yet.
         Assert.That(
-            () => new AnthropicAgentModelClient(Options.Create(new AgentOptions())),
+            () => new AnthropicAgentModelClient(Options.Create(new AgentOptions()), NullLogger<AnthropicAgentModelClient>.Instance),
             Throws.Nothing);
 
     [Test]
@@ -24,7 +25,7 @@ public class AnthropicClientTests
 
         try
         {
-            var client = new AnthropicAgentModelClient(Options.Create(new AgentOptions()));
+            var client = new AnthropicAgentModelClient(Options.Create(new AgentOptions()), NullLogger<AnthropicAgentModelClient>.Instance);
 
             Assert.That(
                 async () => await client.CompleteAsync("system", [], []),