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/Validation/ValidationResultExtensions.cs

@@ -0,0 +1,17 @@
+namespace Novelly.Api.Common.Validation;
+
+public static class ValidationResultExtensions
+{
+    /// <summary>
+    /// The service-level half of "validate again and throw if invalid": callers that reach
+    /// a service directly (agent tools, MCP, tests) skip the API's <see cref="ValidationEndpointFilter"/>,
+    /// so services re-run the same validator and throw rather than act on bad data.
+    /// </summary>
+    public static void ThrowIfInvalid(this ValidationResult result)
+    {
+        if (result.IsInvalid)
+        {
+            throw new ArgumentException(string.Join("; ", result.Errors.Select(e => $"{e.PropertyName}: {e.Message}")));
+        }
+    }
+}