Add course import/export: JSON bundle and direct Word docx upload

- Export any course as a .json bundle (GET /api/admin/courses/:id/export)
- Import a .json bundle as a new course (POST /api/admin/import-course)
- Import one or more unit .docx files directly in the browser — server
  parses each file with mammoth, extracts Week N: headings as lesson
  items and the Unit Quiz section as a graded quiz with answer key
  (POST /api/admin/import-course-docx, multer memory storage)
- Admin UI: Export button on course detail; Import card with JSON and
  Word docs tabs; multi-file picker for docx uploads
- scripts/import-kitchen-course.js: one-time converter that produced
  the Kitchen, Tools & Electrical Skills course JSON bundle (already
  imported as course id 3: 8 units, 44 items)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

package.json

@@ -12,7 +12,8 @@
   "license": "ISC",
   "dependencies": {
     "express": "^5.1.0",
-    "mammoth": "^1.12.0"
+    "mammoth": "^1.12.0",
+    "multer": "^2.2.0"
   },
   "engines": {
     "node": ">=24"