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.

plugins/printedparts/api/routes.py

@@ -250,6 +250,27 @@ def _ledger_write(item, transactiontype, quantitychange, sso, name, reason=None)
         _send_lowstock_alert(item)
 
 
+def _alert_recipients():
+    """Merge selected shopdb users' account emails with the free-text list.
+
+    Empty result means fall back to the site-wide alert_recipients."""
+    from shopdb.api import User
+    recipients = []
+    userids = (Setting.get('printedparts_alert_userids') or '').strip()
+    for rawid in userids.split(','):
+        rawid = rawid.strip()
+        if not rawid.isdigit():
+            continue
+        user = db.session.get(User, int(rawid))
+        if user and user.isactive and user.email:
+            recipients.append(user.email)
+    extra = (Setting.get('printedparts_alert_email') or '').strip()
+    recipients.extend(address.strip() for address in extra.split(',')
+                      if address.strip())
+    # dedupe, order-preserving
+    return list(dict.fromkeys(recipients))
+
+
 def _send_lowstock_alert(item):
     """Best-effort email when an item crosses its low-stock threshold.
 
@@ -265,10 +286,9 @@ def _send_lowstock_alert(item):
             f'<p>Bin: {item.binlocation or "-"}</p>'
             f'<p>Time to print more.</p>')
     try:
-        recipients = (Setting.get('printedparts_alert_email') or '').strip()
+        recipients = _alert_recipients()
         if recipients:
-            send_email([address.strip() for address in recipients.split(',')
-                        if address.strip()], subject, html)
+            send_email(recipients, subject, html)
         else:
             send_alert(subject, html)
     except Exception:

plugins/printedparts/plugin.py

@@ -133,6 +133,9 @@ class PrintedpartsPlugin(BasePlugin):
             ('printedparts_alert_email', '', 'string',
              'Comma-separated low-stock alert recipients; empty uses the '
              'site alert_recipients'),
+            ('printedparts_alert_userids', '', 'string',
+             'Comma-separated shopdb user ids whose account emails receive '
+             'low-stock alerts'),
         ]
         for key, value, valuetype, description in defaults:
             if Setting.get(key) is None: