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.

docs/PLUGIN-HOOKS.md

@@ -9,7 +9,7 @@ The contract is locked in [ADR-001](../docs/adr/ADR-001-asset-as-platform-contra
 The framework declares its contract version in `shopdb/__init__.py`:
 
 ```python
-__contract_version__ = '0.12.0'
+__contract_version__ = '0.13.0'
 ```
 
 Each plugin's `manifest.json` declares the range of contract versions it supports:
@@ -479,6 +479,8 @@ What `shopdb.api` exposes:
 - Import mode: `apply_import_timestamps`, `import_mode_active`,
   `parse_import_datetime`
 - Legacy employee directory: `employee_connection`
+- `User` (0.13.0) - the account model, e.g. resolving alert recipients'
+  emails from selected user ids
 - Mailer (0.12.0): `send_email(to, subject, html, text=None)` and
   `send_alert(subject, html, text=None)` - settings-first, no-op safe when
   email is unconfigured; send_alert targets the site's alert_recipients