chore(phase0): eBay 세트 제거 — 문진·문서 admin/API/lib/CONTENT + cheerio

Delete:
- app/api/questionnaire/ (submit/route.ts)
- app/admin/questionnaire/ (page.tsx)
- app/api/admin/questionnaire/ (route.ts + [id]/route.ts)
- app/admin/documents/ (page.tsx)
- app/api/admin/documents/ ([filename]/route.ts)
- lib/ebay-tools/ (crawler.ts·pricing.ts·ai-analyzer.ts·types.ts)
- CONTENT/ebay-tool-{questionnaire,proposal}.html
- CONTENT/ARCHITECTURE_EBAY_PARTS_TOOL.md

Modify:
- app/admin/components/AdminSidebar.tsx: Remove NAV_ITEMS for /admin/documents & /admin/questionnaire
- package.json: Remove cheerio dependency

Verify: npm test (4 files, 20 tests PASS), npm run build OK

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

package.json

@@ -16,7 +16,6 @@
     "@portone/browser-sdk": "^0.1.3",
     "@supabase/ssr": "^0.5.2",
     "@supabase/supabase-js": "^2.99.0",
-    "cheerio": "^1.2.0",
     "clsx": "^2.1.1",
     "dotenv": "^17.3.1",
     "framer-motion": "^12.38.0",