Add batch character assignment for chapter outline beats

Lets a writer select several beats and add one character to all of them
at once, without disturbing each beat's existing characters. Surfaced
through the REST API, the embedded agent, and the MCP server per the
project's rule that all three share the same service methods.

tests/Novelly.Api.Tests/ServiceTestFixture.cs

@@ -51,7 +51,8 @@ public abstract class ServiceTestFixture
         Chapters = new ChapterService(Db.Context, Tags, ChapterLogs, new CreateChapterRequestValidator(), new UpdateChapterRequestValidator());
         Beats = new BeatService(
             Db.Context, Tags, BeatLogs,
-            new CreateBeatRequestValidator(), new UpdateBeatRequestValidator(), new ReorderBeatsRequestValidator());
+            new CreateBeatRequestValidator(), new UpdateBeatRequestValidator(), new ReorderBeatsRequestValidator(),
+            new AssignCharacterToBeatsRequestValidator());
         Arcs = new CharacterArcService(
             Db.Context, ArcLogs,
             new CreateArcStageRequestValidator(), new UpdateArcStageRequestValidator(), new ReorderArcStagesRequestValidator());