Release 0.9.0

Product version to 0.9.0, frontend in lock-step, Unreleased notes moved into a
dated section per ADR-007. The plugin contract stays at 0.16.0: it moved after
v0.8.1 and is already recorded in this release's notes, and nothing since
touched the contract surface.

A minor rather than a patch: collector behaviour changed in ways an integrator
must know about. A reported machine number no longer becomes the PC's asset
number, it builds a controls link instead; a second PC claiming a machine is
treated as a claim rather than a handover; and a backup revision chain is now
per source PC rather than per asset.

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "shopdb-frontend",
-  "version": "0.8.1",
+  "version": "0.9.0",
   "private": true,
   "type": "module",
   "scripts": {