Five new public endpoints under /api/s/{token}/...:
GET /contents
GET /contents/{folder_id}
GET /file/{file_id}
GET /zip
GET /zip/{folder_id}
All honour the unlock cookie from /verify, so password-protected
folder shares work end-to-end.
Folder/file IDs are validated against the share subtree via a single
ltree containment query (O(log N) on the existing GiST index).
Out-of-scope IDs return 404.
download_shared_file refactored to a Range/304/206/416-aware
serve_share_file helper, shared with the new /file/{file_id}
endpoint. content_disposition extracted from FileHandler so RFC 5987
formatting is identical across auth and share download paths.