Add the printedparts plugin: 3D-printed parts storefront and kiosk

Catalog with photos and append-only print-file revisions, a stock
ledger attributing every take/restock/adjust to a badge-resolved
employee, a touch kiosk (scan bin, scan badge, keypad quantity) with a
deliberately open decrement-only take endpoint, 1x0.5in CODE128 bin
labels, low-stock email alerts to selected users, roles, or addresses,
and stock/consumption/by-person reports with a cache-vs-ledger
reconcile. Ships disabled; enable per site. Contract 0.13.0 adds the
mailer and the User/Role models to the plugin surface.

plugins/printedparts/models/__init__.py

@@ -0,0 +1,11 @@
+"""Printedparts plugin models."""
+
+from .printeditem import (
+    PrintedItem,
+    PrintedItemTransaction,
+    PrintedItemFile,
+    TRANSACTION_TYPES,
+)
+
+__all__ = ['PrintedItem', 'PrintedItemTransaction', 'PrintedItemFile',
+           'TRANSACTION_TYPES']