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.

.editorconfig

@@ -104,6 +104,9 @@ dotnet_style_qualification_for_property = false:suggestion
 dotnet_style_qualification_for_method = false:warning
 dotnet_style_qualification_for_event = false:warning
 
+
+dotnet_diagnostic.CA1873.severity = silent
+
 [*.cs]
 csharp_using_directive_placement = outside_namespace:silent
 csharp_prefer_simple_using_statement = true:suggestion