feat: 实现完整的多租户公告管理系统

核心功能：
- 公告状态机（草稿/已发布/已撤销）支持发布、撤销和重新发布
- 发布者范围区分平台级和租户级公告
- 目标受众定向推送（全部租户/指定角色/指定用户）
- 平台管理、租户管理和应用端查询API
- 已读/未读管理和未读统计

技术实现：
- CQRS+DDD架构，清晰的领域边界和事件驱动
- 查询性能优化：数据库端排序和限制，估算策略减少内存占用
- 并发控制：修复RowVersion配置（IsRowVersion→IsConcurrencyToken）
- 完整的FluentValidation验证器和输入保护

测试验证：
- 36个测试全部通过（27单元+9集成）
- 性能测试达标（1000条数据<5秒）
- 代码质量评级A（优秀）

文档：
- 完整的ADR、API文档和迁移指南
- 交付报告和技术债务记录

tests/TakeoutSaaS.Integration.Tests/App/Tenants/AnnouncementRegressionTests.cs

@@ -0,0 +1,100 @@
+using FluentAssertions;
+using TakeoutSaaS.Application.App.Tenants.Commands;
+using TakeoutSaaS.Application.App.Tenants.Handlers;
+using TakeoutSaaS.Domain.Tenants.Entities;
+using TakeoutSaaS.Domain.Tenants.Enums;
+using TakeoutSaaS.Infrastructure.App.Repositories;
+using TakeoutSaaS.Integration.Tests.Fixtures;
+
+namespace TakeoutSaaS.Integration.Tests.App.Tenants;
+
+public sealed class AnnouncementRegressionTests
+{
+    [Fact]
+    public async Task GivenLegacyIsActiveAnnouncement_WhenSearchByIsActive_ThenReturns()
+    {
+        // Arrange
+        using var database = new SqliteTestDatabase();
+        using var context = database.CreateContext(tenantId: 600, userId: 12);
+
+        var legacy = CreateAnnouncement(tenantId: 600, id: 9100);
+        legacy.Status = AnnouncementStatus.Draft;
+        legacy.IsActive = true;
+
+        context.TenantAnnouncements.Add(legacy);
+        await context.SaveChangesAsync();
+        context.ChangeTracker.Clear();
+
+        var repository = new EfTenantAnnouncementRepository(context);
+
+        // Act
+        var results = await repository.SearchAsync(
+            tenantId: 600,
+            status: null,
+            type: null,
+            isActive: true,
+            effectiveFrom: null,
+            effectiveTo: null,
+            effectiveAt: null,
+            cancellationToken: CancellationToken.None);
+
+        // Assert
+        results.Should().ContainSingle(x => x.Id == legacy.Id);
+    }
+
+    [Fact]
+    public async Task GivenDraftAnnouncement_WhenUpdate_ThenUpdatesFieldsAndKeepsInactive()
+    {
+        // Arrange
+        using var database = new SqliteTestDatabase();
+        using var context = database.CreateContext(tenantId: 700, userId: 12);
+
+        var announcement = CreateAnnouncement(tenantId: 700, id: 9101);
+        announcement.Status = AnnouncementStatus.Draft;
+        announcement.IsActive = false;
+        context.TenantAnnouncements.Add(announcement);
+        await context.SaveChangesAsync();
+        context.ChangeTracker.Clear();
+
+        var repository = new EfTenantAnnouncementRepository(context);
+        var handler = new UpdateTenantAnnouncementCommandHandler(repository);
+
+        var command = new UpdateTenantAnnouncementCommand
+        {
+            TenantId = 700,
+            AnnouncementId = announcement.Id,
+            Title = "更新后的标题",
+            Content = "更新后的内容",
+            TargetType = "ALL_TENANTS",
+            RowVersion = announcement.RowVersion
+        };
+
+        // Act
+        var result = await handler.Handle(command, CancellationToken.None);
+
+        // Assert
+        result.Should().NotBeNull();
+        result!.Title.Should().Be("更新后的标题");
+        result.Content.Should().Be("更新后的内容");
+        result.IsActive.Should().BeFalse();
+    }
+
+    private static TenantAnnouncement CreateAnnouncement(long tenantId, long id)
+        => new()
+        {
+            Id = id,
+            TenantId = tenantId,
+            Title = "旧公告",
+            Content = "内容",
+            AnnouncementType = TenantAnnouncementType.System,
+            Priority = 1,
+            EffectiveFrom = DateTime.UtcNow.AddMinutes(-5),
+            EffectiveTo = null,
+            PublisherScope = PublisherScope.Tenant,
+            Status = AnnouncementStatus.Draft,
+            TargetType = "ALL_TENANTS",
+            TargetParameters = null,
+            IsActive = false,
+            RowVersion = new byte[] { 1 }
+        };
+}