Document the Phase 0 (PLANS.md) split between main and backup DBs in
the example file. .env itself stays out of git per the security rule
in commit.md (line 207) and VIETPRODEV_GUIDELINES.md section 9.

Adds:
- SSO_LOGIN_BACKUP_URL placeholder
- DB_READ_USER / DB_READ_PASSWORD / DB_READ_NAME block
- Comment explaining which block is for write vs read-only fallback

Each operator must still copy .env.example to .env locally and fill in
the actual credentials; only the port numbers and block names live in
the example.
Co-authored-by: Cursor <cursoragent@cursor.com>