Wizard: choose "create tables here" vs "connect your own database" per plugin

Answers the confusion of asking for a DB connection while also offering to
create the tables. Each self-host-capable plugin (employees, usb) now shows a
mode choice; the external connection fields appear only for "connect your own
database". Default is self-hosted (create tables here) - the external path is
the niche/our-site option.

- provisioning_note gains mode_setting; employee_directory_mode + usb_directory_
  mode settings (both default 'selfhosted').
- Wizard renders the radio, shows the note for self-hosted and the config fields
  for external, and saves the chosen mode.

Employees works fully in both modes. USB self-hosted ROUTING is still TODO - the
USB routes read the external cmmc_usb schema; wiring them to the app-owned
tables is the remaining work (tracked).

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

plugins/usb/plugin.py

@@ -58,6 +58,7 @@ class USBPlugin(BasePlugin):
 
     def get_provisioning_note(self) -> Optional[Dict]:
         return {
+            'mode_setting': 'usb_directory_mode',
             'tables': ['usbdevicetypes', 'usbdevices', 'usbcheckouts'],
             'note': ('Enabling this creates USB tracking tables in the shopdb '
                      'database (usbdevicetypes, usbdevices, usbcheckouts) for CMMC '