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.

shopdb/__init__.py

@@ -48,7 +48,7 @@ __contract_version__ = '0.16.0'
 # plugin-contract version above are distinct series with independent
 # bump rules. Not part of the shopdb.api contract surface, so it is
 # not re-exported there.
-__version__ = '0.8.1'
+__version__ = '0.9.0'
 
 
 def create_app(config_name: str = None) -> Flask: