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/NovelAgentService.cs

@@ -28,15 +28,21 @@ public class NovelAgentService(
     private readonly AgentOptions _options = options.Value;
 
     public async Task<IReadOnlyList<ConversationSummaryDto>> ListConversationsAsync(
-        Guid projectId, CancellationToken ct = default) =>
-        await db.Conversations
+        Guid projectId, CancellationToken ct = default)
+    {
+        logger.LogInformation("Listing agent conversations for project {ProjectId}", projectId);
+
+        return await db.Conversations
             .Where(c => c.ProjectId == projectId)
             .OrderByDescending(c => c.UpdatedAt)
             .Select(c => new ConversationSummaryDto(c.Id, c.ProjectId, c.Title, c.Messages.Count, c.UpdatedAt))
             .ToListAsync(ct);
+    }
 
     public async Task<ConversationDto> GetConversationAsync(Guid conversationId, CancellationToken ct = default)
     {
+        logger.LogInformation("Getting agent conversation {ConversationId}", conversationId);
+
         var conversation = await LoadConversationAsync(conversationId, ct);
 
         return new ConversationDto(
@@ -49,6 +55,8 @@ public class NovelAgentService(
 
     public async Task DeleteConversationAsync(Guid conversationId, CancellationToken ct = default)
     {
+        logger.LogInformation("Deleting agent conversation {ConversationId}", conversationId);
+
         var conversation = await LoadConversationAsync(conversationId, ct);
         db.Conversations.Remove(conversation);
         await db.SaveChangesAsync(ct);
@@ -61,6 +69,10 @@ public class NovelAgentService(
     public async Task<AgentTurnDto> SendMessageAsync(
         Guid projectId, SendAgentMessageRequest request, CancellationToken ct = default)
     {
+        logger.LogInformation(
+            "Sending agent message for project {ProjectId}, conversation {ConversationId}, message length {MessageLength}",
+            projectId, request.ConversationId, request.Message.Length);
+
         var conversation = request.ConversationId is { } id
             ? await LoadConversationAsync(id, ct)
             : await StartConversationAsync(projectId, request.Message, ct);
@@ -77,6 +89,8 @@ public class NovelAgentService(
 
         for (var iteration = 0; iteration < _options.MaxIterations; iteration++)
         {
+            logger.LogDebug("Agent iteration {Iteration} for project {ProjectId}", iteration, projectId);
+
             var response = await model.CompleteAsync(systemPrompt, transcript, toolset.Definitions, ct);
 
             foreach (var block in response.Content.OfType<AgentTextBlock>())
@@ -142,6 +156,8 @@ public class NovelAgentService(
     private async Task<AgentMessage> AppendMessageAsync(
         AgentConversation conversation, AgentRole role, string content, string? toolCallsJson, CancellationToken ct)
     {
+        logger.LogDebug("Appending {Role} message to conversation {ConversationId}, content length {ContentLength}", role, conversation.Id, content.Length);
+
         var message = new AgentMessage
         {
             ConversationId = conversation.Id,
@@ -170,8 +186,11 @@ public class NovelAgentService(
     private async Task<AgentConversation> StartConversationAsync(
         Guid projectId, string firstMessage, CancellationToken ct)
     {
+        logger.LogDebug("Starting new agent conversation for project {ProjectId}", projectId);
+
         if (!await db.Projects.AnyAsync(p => p.Id == projectId, ct))
         {
+            logger.LogWarning("Project {ProjectId} not found", projectId);
             throw new NotFoundException(nameof(Project), projectId);
         }
 
@@ -185,11 +204,22 @@ public class NovelAgentService(
         return conversation;
     }
 
-    private async Task<AgentConversation> LoadConversationAsync(Guid conversationId, CancellationToken ct) =>
-        await db.Conversations
+    private async Task<AgentConversation> LoadConversationAsync(Guid conversationId, CancellationToken ct)
+    {
+        logger.LogDebug("Loading agent conversation {ConversationId}", conversationId);
+
+        var conversation = await db.Conversations
             .Include(c => c.Messages)
-            .FirstOrDefaultAsync(c => c.Id == conversationId, ct)
-        ?? throw new NotFoundException(nameof(AgentConversation), conversationId);
+            .FirstOrDefaultAsync(c => c.Id == conversationId, ct);
+
+        if (conversation is null)
+        {
+            logger.LogWarning("AgentConversation {ConversationId} not found", conversationId);
+            throw new NotFoundException(nameof(AgentConversation), conversationId);
+        }
+
+        return conversation;
+    }
 
     /// <summary>
     /// Replays the stored conversation as plain text turns. Tool calls are not replayed —
@@ -208,8 +238,14 @@ public class NovelAgentService(
 
     private async Task<string> BuildSystemPromptAsync(Guid projectId, CancellationToken ct)
     {
-        var project = await db.Projects.FirstOrDefaultAsync(p => p.Id == projectId, ct)
-            ?? throw new NotFoundException(nameof(Project), projectId);
+        logger.LogDebug("Building system prompt for project {ProjectId}", projectId);
+
+        var project = await db.Projects.FirstOrDefaultAsync(p => p.Id == projectId, ct);
+        if (project is null)
+        {
+            logger.LogWarning("Project {ProjectId} not found", projectId);
+            throw new NotFoundException(nameof(Project), projectId);
+        }
 
         var brief = new StringBuilder();
         brief.AppendLine($"Title: {project.Title}");