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/Common/NovellyServiceRegistration.cs

@@ -3,6 +3,7 @@ using Novelly.Api.Agent;
 using Novelly.Api.Beats;
 using Novelly.Api.Chapters;
 using Novelly.Api.Characters;
+using Novelly.Api.Common.Validation;
 using Novelly.Api.Data;
 using Novelly.Api.Projects;
 using Novelly.Api.Questions;
@@ -40,6 +41,8 @@ public static class NovellyServiceRegistration
         services.Configure<AgentOptions>(configuration.GetSection(AgentOptions.SectionName));
         services.AddScoped<IAgentModelClient, AnthropicAgentModelClient>();
 
+        services.AddModelValidatorsFromAssemblyContaining<Program>();
+
         return services;
     }
 }