Printer assignment end to end, and a container that keeps a site's files

Assign printers to a MACHINE and let whichever PC controls it install them, with
the observed half kept strictly apart from the assigned half: a collector report
can never write an assignment row. Adoption is explicit. Adds a singular
relationship type so defaultprinter can mean exactly one, client scripts that
converge a bay's queues and correct a drifted one in place, and a wave gate so a
fleet-wide driver rollout cannot stampede its own share.

On the container side: the compose stack never persisted the instance directory,
so a rebuild discarded plugins.json and every upload. It is a volume now, the
image ships instance/ owned by the app user so the mountpoint is writable, and
the database moves to MySQL 8.4 LTS, which the Windows runbook has recommended
since 8.0 went end of life. The image also could not build at all: the frontend
stage never copied the plugin staging script it runs.

Adds a topology migration guide covering IIS to Docker and back.

shopdb/__init__.py

@@ -63,7 +63,7 @@ __contract_version__ = '0.20.0'
 # 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__ = '0.11.2'
+__version__ = '0.11.3'
 
 
 def create_app(config_name: str = None) -> Flask:
@@ -89,6 +89,13 @@ def create_app(config_name: str = None) -> Flask:
 
     app.config.from_object(config_class)
 
+    # The product and contract versions, in config so a PLUGIN can read them
+    # through current_app instead of importing core (ADR-002 forbids that, and
+    # the contract test enforces it). Core code may still use the module
+    # constants directly.
+    app.config['VERSION'] = __version__
+    app.config['CONTRACT_VERSION'] = __contract_version__
+
     # Load instance config if exists
     app.config.from_pyfile('config.py', silent=True)