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/Agent/NovelAgentToolset.cs

@@ -456,7 +456,7 @@ public class NovelAgentToolset(
                 var characterId = JsonInput.RequiredGuid(input, "character_id");
                 return await OrNotFound(
                     beats.ListForCharacterAsync(characterId, ct),
-                    list => list.Select(b => b.ToCharacterBeatResponse()),
+                    list => list.Select(b => b.ToCharacterBeatResponse(characterId)),
                     "Character",
                     characterId);
             });