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.

tests/Novelly.Api.Tests/BeatServiceTests.cs

@@ -131,7 +131,7 @@ public class BeatServiceTests : ServiceTestFixture
         await Scenes.DeleteAsync(scene.Id);
 
         // The plan outlives a decision about prose — the beat is simply ungrouped.
-        var survivor = await Beats.GetAsync(beat.Id);
+        var survivor = (await Beats.GetAsync(beat.Id))!;
 
         Assert.Multiple(() =>
         {
@@ -148,7 +148,7 @@ public class BeatServiceTests : ServiceTestFixture
             WhatHappened: "Behind the lining of the case.",
             WhatsNext: "She books passage."));
 
-        var renamed = await Beats.UpdateAsync(beat.Id, new UpdateBeatRequest(Title: "She finds it"));
+        var renamed = (await Beats.UpdateAsync(beat.Id, new UpdateBeatRequest(Title: "She finds it")))!;
 
         Assert.Multiple(() =>
         {
@@ -156,7 +156,7 @@ public class BeatServiceTests : ServiceTestFixture
             Assert.That(renamed.WhatsNext, Is.EqualTo("She books passage."));
         });
 
-        var cleared = await Beats.UpdateAsync(beat.Id, new UpdateBeatRequest(WhatsNext: ""));
+        var cleared = (await Beats.UpdateAsync(beat.Id, new UpdateBeatRequest(WhatsNext: "")))!;
 
         Assert.Multiple(() =>
         {