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/api/__init__.py

@@ -44,6 +44,7 @@ from shopdb.core.models import (
     OperatingSystem,
     AssetRelationship,
     RelationshipType,
+    User,
 )
 
 # Response + pagination helpers for plugin API blueprints
@@ -269,6 +270,7 @@ __all__ = [
     'employee_connection',
     'send_email',
     'send_alert',
+    'User',
     # CMMC USB check-in/out database
     'cmmc_usb_connection',
 ]