printedparts stage 11: low-stock email alerts on threshold crossing

Contract 0.12.0: send_email/send_alert join the plugin surface (the
mailer was core-only), PLUGIN-HOOKS and status docs updated, manifest
pins the new floor. The alert fires inside _ledger_write only when a
decrement CROSSES the item's threshold - one alert per depletion,
rearmed by restocking above - and is best-effort after the commit so
mail trouble can never fail a take. Recipients come from
printedparts_alert_email, falling back to the site alert_recipients.
on_enable re-seeds settings idempotently so existing installs pick up
new keys. Crossing/rearm semantics proven by test.

plugins/printedparts/api/routes.py

@@ -228,8 +228,12 @@ def _ledger_write(item, transactiontype, quantitychange, sso, name, reason=None)
     """Append a ledger row and move the cached quantity in ONE commit.
 
     The single-commit invariant is what keeps quantityonhand equal to the
-    ledger sum; every write path must go through here.
+    ledger sum; every write path must go through here. Fires the low-stock
+    alert when this write CROSSES the item's threshold downward - crossing
+    (not being below) is the natural debounce: one alert per depletion, and
+    restocking above the threshold rearms it.
     """
+    quantitybefore = item.quantityonhand
     item.quantityonhand += quantitychange
     db.session.add(PrintedItemTransaction(
         printeditemid=item.printeditemid,
@@ -240,6 +244,37 @@ def _ledger_write(item, transactiontype, quantitychange, sso, name, reason=None)
         reason=reason,
     ))
     db.session.commit()
+    if (quantitychange < 0
+            and quantitybefore > item.lowstockthreshold
+            and item.quantityonhand <= item.lowstockthreshold):
+        _send_lowstock_alert(item)
+
+
+def _send_lowstock_alert(item):
+    """Best-effort email when an item crosses its low-stock threshold.
+
+    Recipients: Setting printedparts_alert_email (comma-separated), falling
+    back to the site's alert_recipients. Never fails the transaction - the
+    ledger write already committed."""
+    from shopdb.api import send_email, send_alert
+    subject = (f'Low stock: {item.itemname} ({item.itemcode}) - '
+               f'{item.quantityonhand} left')
+    html = (f'<p><strong>{item.itemname}</strong> ({item.itemcode}) is down '
+            f'to <strong>{item.quantityonhand}</strong> '
+            f'(threshold {item.lowstockthreshold}).</p>'
+            f'<p>Bin: {item.binlocation or "-"}</p>'
+            f'<p>Time to print more.</p>')
+    try:
+        recipients = (Setting.get('printedparts_alert_email') or '').strip()
+        if recipients:
+            send_email([address.strip() for address in recipients.split(',')
+                        if address.strip()], subject, html)
+        else:
+            send_alert(subject, html)
+    except Exception:
+        import logging
+        logging.getLogger(__name__).exception(
+            'Low-stock alert failed for %s', item.itemcode)
 
 
 @printedparts_bp.route('/items/<int:item_id>/restock', methods=['POST'])