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/Data/Migrations/20260818022141_AddArcStageResultAndBeats.cs

@@ -0,0 +1,61 @@
+using System;
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace Novelly.Api.Data.Migrations
+{
+    /// <inheritdoc />
+    public partial class AddArcStageResultAndBeats : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.RenameColumn(
+                name: "Description",
+                table: "CharacterArcStages",
+                newName: "Result");
+
+            migrationBuilder.CreateTable(
+                name: "ArcStageBeats",
+                columns: table => new
+                {
+                    ArcStagesId = table.Column<Guid>(type: "TEXT", nullable: false),
+                    BeatsId = table.Column<Guid>(type: "TEXT", nullable: false)
+                },
+                constraints: table =>
+                {
+                    table.PrimaryKey("PK_ArcStageBeats", x => new { x.ArcStagesId, x.BeatsId });
+                    table.ForeignKey(
+                        name: "FK_ArcStageBeats_Beats_BeatsId",
+                        column: x => x.BeatsId,
+                        principalTable: "Beats",
+                        principalColumn: "Id",
+                        onDelete: ReferentialAction.Cascade);
+                    table.ForeignKey(
+                        name: "FK_ArcStageBeats_CharacterArcStages_ArcStagesId",
+                        column: x => x.ArcStagesId,
+                        principalTable: "CharacterArcStages",
+                        principalColumn: "Id",
+                        onDelete: ReferentialAction.Cascade);
+                });
+
+            migrationBuilder.CreateIndex(
+                name: "IX_ArcStageBeats_BeatsId",
+                table: "ArcStageBeats",
+                column: "BeatsId");
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropTable(
+                name: "ArcStageBeats");
+
+            migrationBuilder.RenameColumn(
+                name: "Result",
+                table: "CharacterArcStages",
+                newName: "Description");
+        }
+    }
+}

src/Novelly.Api/Data/Migrations/NovelDbContextModelSnapshot.cs

@@ -32,6 +32,21 @@ namespace Novelly.Api.Data.Migrations
                     b.ToTable("BeatCharacters", (string)null);
                 });
 
+            modelBuilder.Entity("BeatCharacterArcStage", b =>
+                {
+                    b.Property<Guid>("ArcStagesId")
+                        .HasColumnType("TEXT");
+
+                    b.Property<Guid>("BeatsId")
+                        .HasColumnType("TEXT");
+
+                    b.HasKey("ArcStagesId", "BeatsId");
+
+                    b.HasIndex("BeatsId");
+
+                    b.ToTable("ArcStageBeats", (string)null);
+                });
+
             modelBuilder.Entity("BeatTag", b =>
                 {
                     b.Property<Guid>("BeatsId")
@@ -398,7 +413,7 @@ namespace Novelly.Api.Data.Migrations
                     b.Property<long>("CreatedAt")
                         .HasColumnType("INTEGER");
 
-                    b.Property<string>("Description")
+                    b.Property<string>("Result")
                         .HasColumnType("TEXT");
 
                     b.Property<int>("SortOrder")
@@ -856,6 +871,21 @@ namespace Novelly.Api.Data.Migrations
                         .IsRequired();
                 });
 
+            modelBuilder.Entity("BeatCharacterArcStage", b =>
+                {
+                    b.HasOne("Novelly.Api.Characters.CharacterArcStage", null)
+                        .WithMany()
+                        .HasForeignKey("ArcStagesId")
+                        .OnDelete(DeleteBehavior.Cascade)
+                        .IsRequired();
+
+                    b.HasOne("Novelly.Api.Beats.Beat", null)
+                        .WithMany()
+                        .HasForeignKey("BeatsId")
+                        .OnDelete(DeleteBehavior.Cascade)
+                        .IsRequired();
+                });
+
             modelBuilder.Entity("BeatTag", b =>
                 {
                     b.HasOne("Novelly.Api.Beats.Beat", null)