feat(filelib): 老师端左栏导航:回收站 + 最近打开(ADR-0031)

- 回收站:listBin(祖先全活跃的已删顶点;管理员/直连 MANAGE 可见)、
  restore(与 D15 对称只清本节点,落审计)、purge(仅管理员,pathIds 枚举
  子树按深度降序分批硬删,绕过 self-FK RESTRICT)
- 最近打开:FileLibRecentVisit 表(filePath='' 兜底 PG 唯一索引),客户端
  成功打开后上报(VIEW 门禁,upsert 刷新),列表 20 条,D8/D15 可见性过滤
- 前端:/app 左栏(文件库/最近打开/回收站);RecentView/BinView;
  GridLibraryView 埋点 + navTarget 跳转(breadcrumb 建栈,role 已捎带)
- 测试:filelib-nav 集成 4 例;全套 79 例绿

docs/adr/0031-filelib-recycle-bin-and-recent-visits.md

@@ -0,0 +1,58 @@
+# ADR 0031: File Library Recycle Bin And Recent-Visit Tracking
+
+## Status
+
+Accepted.
+
+## Context
+
+The teacher app (`/app`) gains a left navigation rail with three entries: 文件库 /
+最近打开 / 回收站. Two of them need semantics that no prior decision covers:
+
+- **回收站 (recycle bin)**: D15 defined soft delete (mark `deletedAt` on the node only;
+  a node is invisible when any ancestor is deleted) but never defined listing, restore,
+  or permanent deletion.
+- **最近打开 (recent visits)**: nothing tracks opens.
+
+## Decision
+
+### Recycle bin
+
+- **List**: shows nodes with `deletedAt != null` whose **ancestors are all active**
+  (the topmost deleted node per branch; descendants of a deleted node are represented
+  by it and not listed separately).
+- **Visibility/auth**: a bin entry is visible to (a) the website administrator, or
+  (b) any actor holding an active MANAGE grant **on the deleted node itself**
+  (grants stay live through soft delete, so this is a plain grant query — no chain
+  walk, no inheritance; the bin is a management surface, not a browsing surface).
+- **Restore** clears `deletedAt` on that node only (D15 symmetry: delete marks one
+  node, restore unmarks one node). The subtree becomes visible again immediately.
+  Same auth as the list entry. Audited (`folder_restore` / `project_restore`).
+- **Permanent delete (彻底删除)** is **website-administrator only**: hard-deletes the
+  node **and its whole subtree** (descendants enumerated via the `pathIds` materialized
+  path, deleted deepest-first because the self-FK is `ON DELETE RESTRICT`), in one
+  transaction, with one audit entry (`node_purge`, detail carries removed count).
+  Grants/settings/export-jobs cascade. There is no recovery; the UI must confirm
+  explicitly.
+
+### Recent visits
+
+- **Model**: `FileLibRecentVisit(organizationId, userId, nodeId, filePath, openedAt)`,
+  unique on `(organizationId, userId, nodeId, filePath)` with `filePath` defaulting to
+  `""` (Postgres unique indexes treat NULLs as distinct). `filePath = ""` means the
+  visit is the node itself (drill into folder/project); non-empty means a file preview
+  inside that project.
+- **Recording is client-driven**: the teacher app POSTs after a successful open
+  (folder drill, project open, file preview). The endpoint requires VIEW on the node
+  (D8: no VIEW → 404, leaking nothing). Upsert semantics: re-opening refreshes
+  `openedAt`. No audit entries — this is a per-user read model, not a权限-sensitive
+  mutation.
+- **List**: the actor's own most recent 20, `openedAt` desc. Entries whose node is
+  deleted **or has any deleted ancestor** are filtered out (D8/D15 visibility holds
+  on every surface). Names are read live from `FileLibNode` (no denormalization).
+
+## Consequences
+
+- No change to existing permission algebra; both features are additive surfaces.
+- The bin deliberately does not offer per-owner bins or inherited-MANAGE visibility —
+  if real usage demands it, that is a new decision.