Self-hosted employee directory (in-app management + CSV import)

Most sites have no external HR database, so add a self-hosted directory mode.

- New employee_directory_mode setting: 'external' (default; read a separate HR
  DB, unchanged) or 'selfhosted' (app-owned table).
- DirectoryEmployee model + directoryemployees table (migration 7d16). to_dict
  emits the same keys the external contract uses (SSO/First_Name/...), so both
  modes share one response shape and the frontend is unchanged.
- Employee search / single / batch lookup branch on the mode.
- Self-hosted-only management endpoints: list, create, update, delete, and CSV
  import (upsert by SSO). Guarded so they only work in self-hosted mode.
- EmployeeDirectory.vue management page (Settings > Locations & Organization):
  table + search + pagination, add/edit/delete, CSV import (file or paste).

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

shopdb/core/api/settings.py

@@ -263,6 +263,13 @@ def build_default_settings():
             'category': 'site',
             'description': 'Set true once the first-run setup wizard has been finished'
         },
+        {
+            'key': 'employee_directory_mode',
+            'value': 'external',
+            'valuetype': 'string',
+            'category': 'site',
+            'description': "Employee directory source: 'external' (a separate HR database) or 'selfhosted' (managed in-app under Employees)"
+        },
         {
             'key': 'site_base_url',
             'value': '',