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.

shopdb/api/__init__.py

@@ -78,6 +78,7 @@ from shopdb.core.services.dualpath import (
 
 # Legacy employee directory lookup (read-only) used by notifications
 from shopdb.utils.employee_db import employee_connection
+from shopdb.utils.mailer import send_email, send_alert
 
 # CMMC USB check-in/out database (read-write) used by the usb plugin
 from shopdb.utils.cmmc_usb_db import cmmc_usb_connection
@@ -266,6 +267,8 @@ __all__ = [
     'parse_import_datetime',
     # Legacy employee directory
     'employee_connection',
+    'send_email',
+    'send_alert',
     # CMMC USB check-in/out database
     'cmmc_usb_connection',
 ]