feat(authz): 重写权限模板分配与班级授权边界

拆分能力、分配范围和业务关系，固定管理员能力并收紧组织级写入口。
补齐内容、课表、设备会话和实时通道授权，兼容旧库分配及历史快照。
重写权限分配预览，按实际可操作范围展示维护入口，合并学生入口并独立学年升级页。
增加授权攻击矩阵及静态门禁，更新隔离回归与开发契约。

验证：授权矩阵 23 段通过，Node 24 下 Web 63/63，类型检查、lint 与文档严格构建通过。

packages/server/prisma/migrations/20261031000000_authz_assignments/migration.sql

@@ -0,0 +1,7 @@
+ALTER TABLE "UserPermissionGroup" ADD COLUMN "scope" TEXT NOT NULL DEFAULT 'TEACHING';
+ALTER TABLE "UserPermissionGroup" ADD COLUMN "scopeValue" TEXT NOT NULL DEFAULT '';
+UPDATE "UserPermissionGroup" SET "scope" = COALESCE((SELECT CASE WHEN g."scope" = 'SELF_CLASS' THEN CASE WHEN g."scopeValue" = 'HEAD' THEN 'HEAD' ELSE 'TEACHING' END ELSE g."scope" END FROM "PermissionGroup" g WHERE g."id" = "groupId"), 'TEACHING'), "scopeValue" = COALESCE((SELECT CASE WHEN g."scope" IN ('GRADE', 'DIVISION') THEN g."scopeValue" ELSE '' END FROM "PermissionGroup" g WHERE g."id" = "groupId"), '');
+UPDATE "PermissionGroup" SET "scope" = CASE WHEN "scopeValue" = 'HEAD' THEN 'HEAD' ELSE 'TEACHING' END, "scopeValue" = '' WHERE "scope" = 'SELF_CLASS';
+UPDATE "PermissionGroup" SET "scope" = 'TEACHING', "scopeValue" = '' WHERE "defaultRole" = 'TEACHER';
+DROP INDEX "UserPermissionGroup_userId_groupId_key";
+CREATE UNIQUE INDEX "UserPermissionGroup_userId_groupId_scope_scopeValue_key" ON "UserPermissionGroup"("userId", "groupId", "scope", "scopeValue");