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/__init__.py

@@ -36,7 +36,7 @@ from .plugins import plugin_manager
 # unattended endpoints (e.g. the GE-Enforce fetch API) can authorize a scoped
 # managed service token without importing core token internals. Additive name
 # on the import surface, minor bump.
-__contract_version__ = '0.11.0'
+__contract_version__ = '0.12.0'
 
 # Product release version (see ADR-007). The product version and the
 # plugin-contract version above are distinct series with independent