printedparts stage 2: models, real 0001 baseline, tables live

PrintedItem (catalog: code, name, image, cached quantityonhand,
per-item threshold, bin) and PrintedItemTransaction (the ledger:
signed quantity change attributed to a badge-resolved employee).
Both registered in PLUGIN_TABLE_OWNERS; 0001 is a post-cutover real
baseline. The migration-guard test learns the new expected head.
Routes are a placeholder ping until the next stage - the scaffold's
list route imported the deleted scaffold model, which surfaces as an
empty 'Migration error' because the alembic env imports the models
package.

plugins/printedparts/models/__init__.py

@@ -1,5 +1,5 @@
 """Printedparts plugin models."""
 
-from .printedparts import Printedparts
+from .printeditem import PrintedItem, PrintedItemTransaction, TRANSACTION_TYPES
 
-__all__ = ['Printedparts']
+__all__ = ['PrintedItem', 'PrintedItemTransaction', 'TRANSACTION_TYPES']