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.

docs/PLUGIN-HOOKS.md

@@ -9,7 +9,7 @@ The contract is locked in [ADR-001](../docs/adr/ADR-001-asset-as-platform-contra
 The framework declares its contract version in `shopdb/__init__.py`:
 
 ```python
-__contract_version__ = '0.11.0'
+__contract_version__ = '0.12.0'
 ```
 
 Each plugin's `manifest.json` declares the range of contract versions it supports:
@@ -479,6 +479,9 @@ What `shopdb.api` exposes:
 - Import mode: `apply_import_timestamps`, `import_mode_active`,
   `parse_import_datetime`
 - Legacy employee directory: `employee_connection`
+- Mailer (0.12.0): `send_email(to, subject, html, text=None)` and
+  `send_alert(subject, html, text=None)` - settings-first, no-op safe when
+  email is unconfigured; send_alert targets the site's alert_recipients
 
 ```python
 from shopdb.api import db, Asset, AssetType, success_response, paginate_query