发信改为事务内每 300 人分批写入，避免全校群发一次插入过大；数据库异常现在返回明确的 503，不再只显示模糊 500。

校级、学院级管理员支持按学院、身份、行政班、教学班、姓名/学号/工号筛选，也可指定最多 500 名收件人。所有结果均由服务端重新校验权限。
接入 CKEditor 5，支持标题、列表、引用和链接；正文扩展为 MySQL longtext。
富文本在收件箱、详情弹窗和已发送记录中统一经 DOMPurify 净化后展示。
页面改用系统现有的冷白、靛蓝、青绿色令牌，新增“选择收件人 → 编辑内容”工作台和移动端适配。

src/Jiaowu.Api/Infrastructure/Persistence/AppDbContext.cs

@@ -980,7 +980,7 @@ public sealed class AppDbContext(DbContextOptions<AppDbContext> options)
         builder.Entity<Notification>(entity =>
         {
             entity.Property(x => x.Title).HasMaxLength(200);
-            entity.Property(x => x.Content).HasMaxLength(1000);
+            entity.Property(x => x.Content).HasColumnType("longtext");
             entity.Property(x => x.LinkUrl).HasMaxLength(300);
             entity.HasIndex(x => new { x.UserId, x.IsRead });
             entity.HasIndex(x => new { x.UserId, x.Category, x.CreatedAt });
@@ -996,7 +996,7 @@ public sealed class AppDbContext(DbContextOptions<AppDbContext> options)
         {
             entity.Property(x => x.SenderName).HasMaxLength(100);
             entity.Property(x => x.Title).HasMaxLength(200);
-            entity.Property(x => x.Content).HasMaxLength(1000);
+            entity.Property(x => x.Content).HasColumnType("longtext");
             entity.Property(x => x.AudienceName).HasMaxLength(200);
             entity.Property(x => x.LinkUrl).HasMaxLength(300);
             entity.HasIndex(x => new { x.SenderUserId, x.CreatedAt });

src/Jiaowu.Api/Infrastructure/Persistence/Migrations/MySql/20260727043019_MessageRichContent.cs

@@ -0,0 +1,54 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace Jiaowu.Api.Infrastructure.Persistence.Migrations.MySql
+{
+    /// <inheritdoc />
+    public partial class MessageRichContent : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.AlterColumn<string>(
+                name: "Content",
+                table: "Notifications",
+                type: "longtext",
+                nullable: false,
+                oldClrType: typeof(string),
+                oldType: "varchar(1000)",
+                oldMaxLength: 1000);
+
+            migrationBuilder.AlterColumn<string>(
+                name: "Content",
+                table: "MessageDispatches",
+                type: "longtext",
+                nullable: false,
+                oldClrType: typeof(string),
+                oldType: "varchar(1000)",
+                oldMaxLength: 1000);
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.AlterColumn<string>(
+                name: "Content",
+                table: "Notifications",
+                type: "varchar(1000)",
+                maxLength: 1000,
+                nullable: false,
+                oldClrType: typeof(string),
+                oldType: "longtext");
+
+            migrationBuilder.AlterColumn<string>(
+                name: "Content",
+                table: "MessageDispatches",
+                type: "varchar(1000)",
+                maxLength: 1000,
+                nullable: false,
+                oldClrType: typeof(string),
+                oldType: "longtext");
+        }
+    }
+}

src/Jiaowu.Api/Infrastructure/Persistence/Migrations/MySql/AppDbContextModelSnapshot.cs

@@ -2366,8 +2366,7 @@ namespace Jiaowu.Api.Infrastructure.Persistence.Migrations.MySql
 
                     b.Property<string>("Content")
                         .IsRequired()
-                        .HasMaxLength(1000)
-                        .HasColumnType("varchar(1000)");
+                        .HasColumnType("longtext");
 
                     b.Property<DateTime>("CreatedAt")
                         .HasColumnType("datetime(6)");
@@ -2413,8 +2412,7 @@ namespace Jiaowu.Api.Infrastructure.Persistence.Migrations.MySql
 
                     b.Property<string>("Content")
                         .IsRequired()
-                        .HasMaxLength(1000)
-                        .HasColumnType("varchar(1000)");
+                        .HasColumnType("longtext");
 
                     b.Property<DateTime>("CreatedAt")
                         .HasColumnType("datetime(6)");