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/BeatEndpoints.cs

@@ -46,7 +46,7 @@ public static class BeatEndpoints
 
         app.MapGet("/api/characters/{characterId:guid}/beats", async (
                 Guid characterId, BeatService service, CancellationToken ct) =>
-                (await service.ListForCharacterAsync(characterId, ct))?.Select(b => b.ToCharacterBeatResponse()).ToList().ToApiResult())
+                (await service.ListForCharacterAsync(characterId, ct))?.Select(b => b.ToCharacterBeatResponse(characterId)).ToList().ToApiResult())
             .WithTags("Beats")
             .WithSummary("Every beat this character appears in, in manuscript order.");