Add protected admin submission exports

[Spbd1]

Motivation

• Provide researchers a secure way to retrieve server-submitted study data from the deployed server using protected admin API endpoints and a simple CLI export.
• Prevent exposing admin functionality in the participant UI while enforcing server-only token validation and safe error behavior.

Description

• Add a server-only admin auth helper validateAdminRequest that reads ADMIN_EXPORT_TOKEN, returns a safe 500 setup error when the token is not configured, returns 401 for missing/invalid bearer tokens, and uses SHA-256 digest + timingSafeEqual for constant-time comparison without exposing token values (lib/adminAuth.server.ts).
• Implement GET /api/admin/submissions returning JSON with ok, items, and nextCursor and cursor pagination based on submittedAt + id with limit default 100 and max 500 (app/api/admin/submissions/route.ts, lib/adminSubmissions.ts).
• Implement GET /api/admin/submissions.csv returning text/csv with one row per submission and proper CSV escaping, outputting empty strings for missing fields and including the requested columns from submission/game/survey/background/metrics (app/api/admin/submissions.csv/route.ts, lib/adminSubmissions.ts).
• Add a CLI export script scripts/export-submissions.ts that reads APP_BASE_URL and ADMIN_EXPORT_TOKEN, calls /api/admin/submissions, creates exports/, and writes exports/submissions-[timestamp].json; add npm script export:submissions (package.json).
• Update README.md with curl examples for JSON and CSV exports and a usage example for the export script.

Testing

• Ran npm run typecheck but it could not complete because npm install failed to fetch @prisma/client from the registry, so typechecking failed due to missing dependencies/types (registry 403); result: not completed.
• Attempted npm run lint and npm run build but both could not run because dependencies (including next) were not installed; result: not completed.
• Verified with a repository search that admin endpoints are not referenced in the participant UI using rg/grep, confirming the admin routes are not linked from frontend code; result: success.

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Codex Task