Employees + USB default-disabled with an enable-time provisioning note

Both plugins provision extra tables, so they now install disabled and explain
themselves before a site opts in.

- Plugin contract gains get_provisioning_note() -> {tables, note, docs}.
  Employees and USB implement it (what tables get created in shopdb, how they
  are referenced, link to the schema README; USB references the captured
  DLP/reminder plans).
- Manifest default_enabled=false for employees + usb; the plugins list API
  returns provisioning_note + default_enabled; install now registers a plugin
  disabled when default_enabled is false.
- Setup wizard Features step renders the provisioning note the moment a plugin
  with one is enabled.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

shopdb/plugins/base.py

@@ -74,6 +74,20 @@ class BasePlugin(ABC):
         """Return dict of service name -> service class."""
         return {}
 
+    def get_provisioning_note(self) -> Optional[Dict]:
+        """Transparency note shown when a site enables this plugin.
+
+        Return None for plugins that need no special setup. For plugins that
+        create extra tables (e.g. a self-hosted directory or USB tables), return:
+          {
+            'tables': ['directoryemployees', ...],   # created in the shopdb DB
+            'note':   'Plain-language what/why.',
+            'docs':   'plugins/<name>/README.md',     # where the schema lives
+          }
+        The setup wizard shows this the moment the plugin is checked.
+        """
+        return None
+
     def get_config_schema(self) -> List[Dict]:
         """Declare the config fields this plugin needs, for the setup wizard.