已完成教学任务归档纠错功能：

已归档任务新增“撤销归档”和“删除”，支持单条及批量操作。
撤销后恢复为“已发布”状态。
已发布任务仍不可直接删除；被排课、选课、成绩或考试引用的归档任务也会拒绝删除并明确提示。
状态与操作文案统一为“归档”

src/Jiaowu.Api/Controllers/TeachingTasksController.cs

@@ -228,10 +228,14 @@ public sealed class TeachingTasksController(
             .FirstOrDefaultAsync(x => x.Id == id, cancellationToken);
         if (task is null) return NotFound();
         if (!CanManage(task.Course!)) return Forbid();
-        if (task.Status != TeachingTaskStatus.Draft)
-            return ConflictProblem("仅草稿教学任务可以删除。");
+        if (task.Status is not (TeachingTaskStatus.Draft or TeachingTaskStatus.Closed))
+            return ConflictProblem("仅草稿或已归档的教学任务可以删除。");
         db.TeachingTasks.Remove(task);
-        return await SaveAsync(id, false, cancellationToken);
+        return await SaveAsync(
+            id,
+            false,
+            cancellationToken,
+            "教学任务已被排课、选课、成绩或考试等业务引用，不能删除；如需继续使用，可撤销归档。");
     }
 
     [HttpPost("{id:guid}/publish")]
@@ -263,11 +267,25 @@ public sealed class TeachingTasksController(
         if (task is null) return NotFound();
         if (!CanManage(task.Course!)) return Forbid();
         if (task.Status != TeachingTaskStatus.Published)
-            return ConflictProblem("只有已发布的教学任务可以结课。");
+            return ConflictProblem("只有已发布的教学任务可以归档。");
         task.Status = TeachingTaskStatus.Closed;
         return await SaveAsync(id, false, cancellationToken);
     }
 
+    [HttpPost("{id:guid}/unarchive")]
+    public async Task<ActionResult> Unarchive(Guid id, CancellationToken cancellationToken)
+    {
+        var task = await ScopedTasks()
+            .Include(x => x.Course)
+            .FirstOrDefaultAsync(x => x.Id == id, cancellationToken);
+        if (task is null) return NotFound();
+        if (!CanManage(task.Course!)) return Forbid();
+        if (task.Status != TeachingTaskStatus.Closed)
+            return ConflictProblem("只有已归档的教学任务可以撤销归档。");
+        task.Status = TeachingTaskStatus.Published;
+        return await SaveAsync(id, false, cancellationToken);
+    }
+
     [HttpPost("batch")]
     public async Task<ActionResult> Batch(
         TeachingTaskBatchRequest request,
@@ -305,9 +323,10 @@ public sealed class TeachingTasksController(
                 break;
             }
             case TeachingTaskBatchOperation.Delete:
-                if (tasks.Any(x => x.Status != TeachingTaskStatus.Draft))
+                if (tasks.Any(x =>
+                        x.Status is not (TeachingTaskStatus.Draft or TeachingTaskStatus.Closed)))
                     return ConflictProblem(
-                        $"教学任务“{tasks.First(x => x.Status != TeachingTaskStatus.Draft).Name}”不是草稿，整批未删除。");
+                        $"教学任务“{tasks.First(x => x.Status is not (TeachingTaskStatus.Draft or TeachingTaskStatus.Closed)).Name}”不是草稿或已归档状态，整批未删除。");
                 db.TeachingTasks.RemoveRange(tasks);
                 break;
             case TeachingTaskBatchOperation.Archive:
@@ -316,6 +335,12 @@ public sealed class TeachingTasksController(
                         $"教学任务“{tasks.First(x => x.Status != TeachingTaskStatus.Published).Name}”不是已发布状态，整批未归档。");
                 foreach (var task in tasks) task.Status = TeachingTaskStatus.Closed;
                 break;
+            case TeachingTaskBatchOperation.Unarchive:
+                if (tasks.Any(x => x.Status != TeachingTaskStatus.Closed))
+                    return ConflictProblem(
+                        $"教学任务“{tasks.First(x => x.Status != TeachingTaskStatus.Closed).Name}”不是已归档状态，整批未撤销归档。");
+                foreach (var task in tasks) task.Status = TeachingTaskStatus.Published;
+                break;
             default:
                 return ValidationProblem("不支持的批量操作。");
         }
@@ -664,7 +689,8 @@ public sealed class TeachingTasksController(
     private async Task<ActionResult> SaveAsync(
         Guid id,
         bool created,
-        CancellationToken cancellationToken)
+        CancellationToken cancellationToken,
+        string conflictMessage = "教学任务编号重复，或关联数据已失效。")
     {
         try
         {
@@ -673,7 +699,7 @@ public sealed class TeachingTasksController(
         }
         catch (DbUpdateException)
         {
-            return ConflictProblem("教学任务编号重复，或关联数据已失效。");
+            return ConflictProblem(conflictMessage);
         }
     }
 
@@ -739,5 +765,6 @@ public enum TeachingTaskBatchOperation
 {
     Publish = 1,
     Delete = 2,
-    Archive = 3
+    Archive = 3,
+    Unarchive = 4
 }