fix: clarify database selection during setup

[atomantic]

Summary

Correct the database setup guide's claim that a healthy native database is always detected first. With Docker available, an unset PGMODE selects Docker instead, potentially pointing a reader at a separate empty database.

• Evidence: docs/STORAGE.md:153 and its former readiness-first sequence at line 174 contradict scripts/setup-db.js:51, scripts/setup-db.js:285, and scripts/setup-db.js:357.
• Document explicit native selection, the conditional native fallback, and the distinction between selecting a database and migrating records.

Test plan

• cd server && npm test -- ../scripts/setup-db.test.js — 8 tests passed.
• Checked the documented branches against setup-db.js and the Docker Compose configuration; no database provisioning was executed.
• git diff --check passed.