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>

plugins/usb/plugin.py

@@ -56,6 +56,20 @@ class USBPlugin(BasePlugin):
         """Return list of SQLAlchemy model classes."""
         return [USBDeviceType, USBDevice, USBCheckout]
 
+    def get_provisioning_note(self) -> Optional[Dict]:
+        return {
+            'tables': ['usbdevicetypes', 'usbdevices', 'usbcheckouts'],
+            'note': ('Enabling this creates USB tracking tables in the shopdb '
+                     'database (usbdevicetypes, usbdevices, usbcheckouts) for CMMC '
+                     'removable-media check-in/out. Devices, check-in/out events, '
+                     'and per-event log ids live here. Planned build-out: a USB-ID '
+                     'standard, overdue-device email reminders, and DLP/Fabric '
+                     'approval integration (see the captured design notes). A site '
+                     'with an existing cmmc_usb database can use external mode '
+                     'instead.'),
+            'docs': 'plugins/usb/README.md',
+        }
+
     def get_config_schema(self) -> List[Dict]:
         """CMMC USB check-in/out database connection. Host/name/user are settings
         the wizard can edit; the password stays in .env (emitted, not stored)."""