Auth: Bootstrap admin script

[EricGrill]

Auth System — Step 5 (Backend)

Create the first admin user (can't use invite flow — no one to invite from).

Implementation:

• backend/scripts/create-admin.js CLI script
• Usage: node scripts/create-admin.js --email admin@cafejusto.app --password "..."
• Creates user with admin role (farm_id = NULL)
• Alternatively: auto-create from BOOTSTRAP_ADMIN_EMAIL + BOOTSTRAP_ADMIN_PASSWORD env vars if users table is empty on startup
• Refuse to run if admin already exists (prevent duplicates)

Depends on: DB migration
Design doc: docs/AUTH.md