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>

frontend/src/router/routes/core.js

@@ -146,6 +146,12 @@ export default [
     component: () => import('../../views/settings/CustomFieldsList.vue'),
     meta: { requiresAuth: true, requiresAdmin: true }
   },
+  {
+    path: 'settings/employeedirectory',
+    name: 'employee-directory',
+    component: () => import('../../views/settings/EmployeeDirectory.vue'),
+    meta: { requiresAuth: true, requiresAdmin: true }
+  },
   {
     path: 'settings/system',
     name: 'system-settings',