feat: 评论/帖子编辑历史与编辑标记，内容锁定

- 新增 smartDiff 差异计算与 EditHistoryModal 编辑历史弹窗，替换原 CommentEditHistoryModal
- 新增 PostEditedMark 编辑标记展示
- 新增内容锁定机制（content_lock）防止并发编辑冲突
- 审核与通知服务适配

backend/service/content_lock_test.go

@@ -0,0 +1,74 @@
+package service
+
+import (
+	"testing"
+	"time"
+
+	"github.com/freefire/jiang13-bbs/model"
+)
+
+// TestEditLocked 内容自动编辑锁定规则（帖子与评论共用）：
+// staff 豁免、hours<=0 关闭、以创建时间为基准计时
+func TestEditLocked(t *testing.T) {
+	now := time.Date(2026, 9, 25, 12, 0, 0, 0, time.Local)
+	createdAt := now.Add(-48 * time.Hour) // 两天前创建
+
+	cases := []struct {
+		name      string
+		createdAt time.Time
+		isStaff   bool
+		hours     int
+		now       time.Time
+		want      bool
+	}{
+		{"未启用", createdAt, false, 0, now, false},
+		{"负值视为关闭", createdAt, false, -1, now, false},
+		{"超过时限锁定", createdAt, false, 24, now, true},
+		{"恰好到期未锁（严格大于）", createdAt, false, 48, now, false},
+		{"未到期不锁", createdAt, false, 72, now, false},
+		{"staff 超时也豁免", createdAt, true, 1, now, false},
+		{"staff 管理员角色豁免", createdAt, model.IsStaff(model.RoleAdmin), 1, now, false},
+		{"普通用户非豁免", createdAt, model.IsStaff(model.RoleUser), 1, now, true},
+		{"板块管理员豁免", createdAt, model.IsStaff(model.RoleBoardAdmin), 1, now, false},
+		{"刚创建且时限极短不锁", now, false, 1, now, false},
+	}
+
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			if got := editLocked(c.createdAt, c.isStaff, c.hours, c.now); got != c.want {
+				t.Fatalf("editLocked() = %v, want %v", got, c.want)
+			}
+		})
+	}
+}
+
+// TestIsNecroReply 旧帖判定：最后回复时间（无回复回落发帖时间）距 now 超过阈值。
+// staff 豁免、规则开关、回落基准由调用方负责，不在本 helper 内。
+func TestIsNecroReply(t *testing.T) {
+	now := time.Date(2026, 9, 25, 12, 0, 0, 0, time.Local)
+	lastReplyAt := now.Add(-48 * time.Hour) // 最后回复在两天前
+
+	cases := []struct {
+		name        string
+		lastReplyAt time.Time
+		afterHours  int
+		now         time.Time
+		want        bool
+	}{
+		{"阈值关闭", lastReplyAt, 0, now, false},
+		{"阈值负值视为关闭", lastReplyAt, -1, now, false},
+		{"超过阈值触发", lastReplyAt, 24, now, true},
+		{"恰好到期不触发（严格大于）", lastReplyAt, 48, now, false},
+		{"未到期不触发", lastReplyAt, 72, now, false},
+		{"无回复回落发帖时间同理（基准即发帖时间）", now.Add(-72 * time.Hour), 48, now, true},
+		{"刚回复不久不触发", now.Add(-time.Hour), 2, now, false},
+	}
+
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			if got := IsNecroReply(c.lastReplyAt, c.afterHours, c.now); got != c.want {
+				t.Fatalf("IsNecroReply() = %v, want %v", got, c.want)
+			}
+		})
+	}
+}