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.

src/Novelly.Api/Beats/Beat.cs

@@ -19,6 +19,8 @@ public class Beat
 
     public List<Character> Characters { get; set; } = [];
 
+    public List<CharacterArcStage> ArcStages { get; set; } = [];
+
     public string? WhatHappened { get; set; }
 
     public string? WhatsNext { get; set; }