trash: a delete can be undone

`x`, `dd`, `:delete` and `:rmspace` no longer end anything. What they
delete is moved, whole, under one hidden root, and `u` puts the last of it
back where it was. `:trash` lists everything deleted, the last first:
Enter or `r` restores, `x` deletes one for good and `:trash purge` empties
it, each asking first. Something whose folder has gone since comes back to
the root of its space; something whose space is in the trash too waits for
the space. The daily sweep purges what has waited WORKSPACE_TRASH_DAYS (30),
and only then are its files deleted.

While it waits, a trashed node is out of the tree, the search, the agenda,
the mails, the SFTP mount, the export and the readable files/ view, and a
shared document answers 404 until it comes back with the same link. `rm`
over the mount trashes too; a rename onto a taken name still replaces what
was there, so editor saves do not fill the trash.

migrations/0019_trash.sql

@@ -0,0 +1,22 @@
+-- A delete puts a node in the trash rather than ending it. The trash is one
+-- hidden root: a folder, not a space, so nothing that lists the spaces or
+-- walks down from them ever reaches it, and a trashed node — moved under it
+-- whole, with everything inside — drops out of the tree, the search, the
+-- mount, the export and the readable mirror at once. The slug starts with a
+-- dot, which no name ever slugifies to, so no space can take it.
+INSERT INTO nodes (parent_id, kind, name, slug, role, created_at, updated_at)
+VALUES (NULL, 'folder', 'trash', '.trash', 'trash', strftime('%s', 'now'), strftime('%s', 'now'));
+
+-- One row per thing deleted, not per node under it: a folder comes back with
+-- what was in it. Where it came from is kept by id, to go back to, and as the
+-- path it had, to show once that folder is gone too. A trashed space has no
+-- parent to go back to; it goes back to the root.
+CREATE TABLE trash (
+    node_id     INTEGER PRIMARY KEY REFERENCES nodes (id) ON DELETE CASCADE,
+    from_parent INTEGER,
+    from_space  INTEGER,
+    from_path   TEXT    NOT NULL,
+    deleted_at  INTEGER NOT NULL
+);
+
+CREATE INDEX idx_trash_deleted ON trash (deleted_at);