integrity: one write, one transaction, and nothing left behind

Node update, delete and move each ran as separate statements, with the
slug chosen outside the write that took it. They are now one transaction
each, opened with BEGIN IMMEDIATE so a read-then-write cannot fail with
SQLITE_BUSY halfway. Creating a node, a space or a file does the same.

A delete is one function, delete_subtree, used by the API and the SFTP
mount alike: SFTP deletes used to skip the media cleanup and orphan the
files a document embedded. Rows go in the transaction, bytes after the
commit. Deleting a media row that backs a file in the tree is refused;
the media browser marks those files and stops counting them as
documents that embed them.

A space can no longer be moved into a folder. The diary is found by a
role column (migration 0013) instead of its slug, so renaming it no
longer breaks the reminder or the agenda, and it cannot be deleted. The
same migration adds the unique index root slugs never had.

A card is done exactly when it is in the last list: a done patch now
moves the card, a card made in the done list is stamped, and adding or
deleting lists settles the board. Seeding a fresh board is atomic, and
a card links only to a document.

Uploads are fsynced before the rename, and zip entries are written aside
and renamed like any other upload. A sweep at start and once a day
removes leftover .part files, files no row knows, and uploads nothing
has embedded for a week.

Before a new build migrates the database it writes a VACUUM INTO copy,
diary.db.pre-<version>, since migrations only go forward.

web/src/lib/api.ts

@@ -23,6 +23,8 @@ export interface NodeSummary {
   excerpt: string
   position: number
   has_board: boolean
+  /** Spaces the app has to tell apart: `diary`, or nothing. */
+  role: string | null
   child_count: number
   created_at: number
   updated_at: number
@@ -45,13 +47,13 @@ export interface WorkspaceNode extends NodeSummary {
   path: Crumb[]
 }
 
-/** A public key allowed to mount the workspace over SFTP. */
 /** A space as the overview shows it: itself, and what was last written in it. */
 export interface SpaceOverview {
   space: NodeSummary
   recent: NodeSummary[]
 }
 
+/** A public key allowed to mount the workspace over SFTP. */
 export interface SshKey {
   id: number
   name: string
@@ -130,6 +132,8 @@ export interface MediaFile {
   encrypted: boolean
   /** Every document that embeds this file — a file may be used by several. */
   node_ids: number[]
+  /** The file node it is, when it sits in the tree; deleted from there. */
+  file_node: number | null
 }
 
 /** The vault, as `/api/vault` reports it. */