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/manifest.json

@@ -5,7 +5,7 @@
     "display_name": "3D Printed Parts",
     "author": "",
     "dependencies": ["employees"],
-    "core_version": ">=0.11.0,<1.0.0",
+    "core_version": ">=0.12.0,<1.0.0",
     "api_prefix": "/api/printedparts",
     "default_enabled": false
 }