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/plugin.py

@@ -103,6 +103,12 @@ class PrintedpartsPlugin(BasePlugin):
             self._seed_settings()
             logger.info('Printedparts plugin installed')
 
+    def on_enable(self, app: Flask) -> None:
+        # Idempotent re-seed so settings added in later versions reach sites
+        # that installed earlier (enable runs on every upgrade cycle).
+        with app.app_context():
+            self._seed_settings()
+
     def _seed_settings(self) -> None:
         defaults = [
             ('printedparts_code_prefix', '3DP', 'string',
@@ -111,6 +117,9 @@ class PrintedpartsPlugin(BasePlugin):
              'Default low-stock threshold for new items'),
             ('printedparts_unknown_badge', 'deny', 'string',
              'Kiosk policy when a badge resolves to no employee: allow or deny'),
+            ('printedparts_alert_email', '', 'string',
+             'Comma-separated low-stock alert recipients; empty uses the '
+             'site alert_recipients'),
         ]
         for key, value, valuetype, description in defaults:
             if Setting.get(key) is None: