Group character beats into arc-stage sections; reciprocal relationship types

Arc stages now group the beats that establish or pay off that stage of a
character's arc (many-to-many via ArcStageBeats), and carry a Result field
(renamed from Description) describing what the stage results in for the
character. Assigning a beat to a stage moves it out of any other stage of
the same character. New endpoint POST /api/arc-stages/{id}/beats, MCP tool
set_arc_stage_beats, and frontend grouping UI in CharacterArc/CharacterBeats.

Also records a relationship's reciprocal type so both characters' dossiers
show the correct direction (e.g. "sister" / "brother") instead of mirroring
the same label.

tests/Novelly.Api.Tests/ServiceTestFixture.cs

@@ -72,7 +72,8 @@ public abstract class ServiceTestFixture
             new AssignCharacterToBeatsRequestValidator(), new MoveBeatsRequestValidator());
         Arcs = new CharacterArcService(
             Db.Context, Access, ArcLogs,
-            new CreateArcStageRequestValidator(), new UpdateArcStageRequestValidator(), new ReorderArcStagesRequestValidator());
+            new CreateArcStageRequestValidator(), new UpdateArcStageRequestValidator(), new ReorderArcStagesRequestValidator(),
+            new SetArcStageBeatsRequestValidator());
         Questions = new OpenQuestionService(
             Db.Context, Access, QuestionLogs,
             new CreateOpenQuestionRequestValidator(), new UpdateOpenQuestionRequestValidator(), new ResolveOpenQuestionRequestValidator());