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/BeatService.cs

@@ -68,6 +68,7 @@ public class BeatService(
 
         var beats = await db.Beats
             .Include(b => b.Chapter)
+            .Include(b => b.ArcStages)
             .Where(b => b.Characters.Any(c => c.Id == characterId))
             .ToListAsync(ct);