printedparts stage 1: scaffold, no AssetType, manifest per spec

flask plugin new output, minus the scaffold's AssetType seeding:
printed parts are quantity-based consumables, not ADR-001 assets.
on_install seeds the three plugin settings instead. Manifest pins
core >=0.11.0, depends on employees (badge name resolution), ships
disabled until a site opts in.

plugins/printedparts/schemas/__init__.py

@@ -0,0 +1,6 @@
+"""Printedparts plugin schemas (marshmallow).
+
+Add schema classes here when you need request/response validation
+beyond the simple to_dict() output. The framework wires marshmallow
+into the response helpers; see docs/PLUGIN-HOOKS.md for details.
+"""