Plugin system: fix PluginMeta core_version default + add /api/plugins

- PluginMeta.core_version defaulted to >=1.0.0, which would reject the current
  0.2.0 framework for any plugin relying on the default. Set to >=0.2.0,<1.0.0.
- Add GET /api/plugins introspection: lists loaded plugins (name, version,
  core_version, api_prefix, dependencies) + the framework contract version.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

shopdb/__init__.py

@@ -80,6 +80,7 @@ CORE_BLUEPRINT_NAMES = (
     'auth',
     'assets',
     'machinetypes',
+    'plugins',
     'vendors',
     'models',
     'businessunits',