printedparts stage 13: pick alert recipients from shopdb users

Contract 0.13.0 puts the User model on the plugin surface. The
settings page gains a checkbox picker over the user list; selected
users receive low-stock alerts at their account email, merged and
deduped with the free-text address list, inactive accounts skipped,
site alert_recipients still the fallback when both are empty.

shopdb/__init__.py

@@ -36,7 +36,7 @@ from .plugins import plugin_manager
 # unattended endpoints (e.g. the GE-Enforce fetch API) can authorize a scoped
 # managed service token without importing core token internals. Additive name
 # on the import surface, minor bump.
-__contract_version__ = '0.12.0'
+__contract_version__ = '0.13.0'
 
 # Product release version (see ADR-007). The product version and the
 # plugin-contract version above are distinct series with independent