feat: make ROOT_FOLDER_ID configurable via env

[mfscpayload-690]

Summary

Make Google Drive root folder selection configurable via environment variable instead of hardcoding it in code.

Problem

ROOT_FOLDER_ID is currently hardcoded in netlify/functions/api.js.
This makes environment-specific deployments harder and requires code changes + redeploys for folder updates.

Proposed Change

• Read root folder ID from env var:
  • ROOT_FOLDER_ID
• Keep a backward-compatible fallback to current hardcoded ID (optional, for transition).
• Return a clear startup/runtime error if ROOT_FOLDER_ID is missing and no fallback is allowed.
• Document the variable in:
  • .env.example
  • README.md

Acceptance Criteria

• API uses process.env.ROOT_FOLDER_ID for /api/folders.
• .env.example includes ROOT_FOLDER_ID=... with description.
• README setup section includes this variable.
• App works in local + Netlify when ROOT_FOLDER_ID is configured.
• No behavior regression for existing endpoints (/api/folders, /api/files/:folderId, /api/search).

Out of Scope

• Changing Drive API query logic beyond root folder source.
• Adding user-level auth or per-user folder routing.