Stop throwing for not-found; add Guard and request validation

Not-found lookups return null/false instead of throwing NotFoundException
across all services — a missing row is expected control flow, not an
exceptional condition. NotFoundException stays for embedded precondition
checks inside mutations (missing parent, invalid foreign reference).

Guard (copied from mic-check) enforces required arguments at the top of
every service method. A ported IModelValidator<T> framework validates
every request DTO at the API layer via a new ValidationEndpointFilter,
returning a 400 with field-level messages; services re-run the same
validator and throw for direct callers that bypass the API.

Endpoints translate null/false into 404 via a new ToApiResult() helper.
The agent toolset boundary translates the same nullable/bool results
into the tool-error text the model already expected.

src/Novelly.Api/Agent/AgentConversation.cs

@@ -5,16 +5,16 @@ namespace Novelly.Api.Agent;
 /// <summary>A chat thread between the writer and the embedded agent, scoped to one project.</summary>
 public class AgentConversation
 {
-    public Guid Id { get; set; } = Guid.NewGuid();
-    public Guid ProjectId { get; set; }
-    public Project? Project { get; set; }
+    public Guid Id { get; init; } = Guid.NewGuid();
+    public Guid ProjectId { get; init; }
+    public Project? Project { get; init; }
 
-    public string Title { get; set; } = "New conversation";
+    public string Title { get; init; } = "New conversation";
 
-    public DateTimeOffset CreatedAt { get; set; } = DateTimeOffset.UtcNow;
+    public DateTimeOffset CreatedAt { get; init; } = DateTimeOffset.UtcNow;
     public DateTimeOffset UpdatedAt { get; set; } = DateTimeOffset.UtcNow;
 
-    public List<AgentMessage> Messages { get; set; } = [];
+    public List<AgentMessage> Messages { get; init; } = [];
 }
 
 /// <summary>
@@ -24,26 +24,26 @@ public class AgentConversation
 /// </summary>
 public class AgentMessage
 {
-    public Guid Id { get; set; } = Guid.NewGuid();
-    public Guid ConversationId { get; set; }
-    public AgentConversation? Conversation { get; set; }
+    public Guid Id { get; init; } = Guid.NewGuid();
+    public Guid ConversationId { get; init; }
+    public AgentConversation? Conversation { get; init; }
 
-    public AgentRole Role { get; set; }
+    public AgentRole Role { get; init; }
 
     /// <summary>
     /// Position in the conversation, 0-based. Timestamps are not enough to order a
     /// transcript: a fast turn can produce two messages inside the same tick.
     /// </summary>
-    public int Sequence { get; set; }
+    public int Sequence { get; init; }
 
     /// <summary>The visible text of the turn.</summary>
-    public string Content { get; set; } = string.Empty;
+    public string Content { get; init; } = string.Empty;
 
     /// <summary>
     /// JSON array of <c>{ name, input, result }</c> objects describing tool calls made
     /// during this turn. Null on user turns and on assistant turns that used no tools.
     /// </summary>
-    public string? ToolCallsJson { get; set; }
+    public string? ToolCallsJson { get; init; }
 
-    public DateTimeOffset CreatedAt { get; set; } = DateTimeOffset.UtcNow;
+    public DateTimeOffset CreatedAt { get; init; } = DateTimeOffset.UtcNow;
 }