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.Api/Agent/NovelAgentToolset.cs

@@ -33,7 +33,8 @@ public class NovelAgentToolset(
     BeatService beats,
     SceneService scenes,
     TagService tags,
-    OpenQuestionService questions)
+    OpenQuestionService questions,
+    ILogger<NovelAgentToolset> logger)
 {
     private static readonly JsonSerializerOptions SerializerOptions = new()
     {
@@ -57,24 +58,31 @@ public class NovelAgentToolset(
     {
         if (!ByName.TryGetValue(name, out var tool))
         {
+            logger.LogWarning("Agent requested unknown tool {Tool}", name);
             return new AgentToolResult($"No such tool: '{name}'.", true);
         }
 
+        logger.LogDebug("Running tool {Tool} for project {ProjectId}", name, projectId);
+
         try
         {
             var result = await tool.Handler(projectId, input, ct);
+            logger.LogDebug("Tool {Tool} for project {ProjectId} succeeded", name, projectId);
             return new AgentToolResult(JsonSerializer.Serialize(result, SerializerOptions), false);
         }
         catch (NotFoundException ex)
         {
+            logger.LogWarning(ex, "Tool {Tool} for project {ProjectId} failed: not found", name, projectId);
             return new AgentToolResult(ex.Message, true);
         }
         catch (ArgumentException ex)
         {
+            logger.LogWarning(ex, "Tool {Tool} for project {ProjectId} failed: invalid argument", name, projectId);
             return new AgentToolResult(ex.Message, true);
         }
         catch (InvalidOperationException ex)
         {
+            logger.LogWarning(ex, "Tool {Tool} for project {ProjectId} failed: invalid operation", name, projectId);
             return new AgentToolResult(ex.Message, true);
         }
     }