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/ApiResultExtensions.cs

@@ -0,0 +1,12 @@
+namespace Novelly.Api.Common;
+
+public static class ApiResultExtensions
+{
+    /// <summary>
+    /// A missing entity is not exceptional, so lookups return null instead of throwing.
+    /// This is where that null finally becomes an HTTP 404 — the one place the API layer
+    /// needs to know about it.
+    /// </summary>
+    public static IResult ToApiResult<T>(this T? value) where T : class =>
+        value is null ? Results.NotFound() : Results.Ok(value);
+}