feat(env): align .env.example with two-postgres topology

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>

.env.example

@@ -11,6 +11,7 @@ PROJECT_VERSION=1.0.0
 # ─────────────────────────────────────────────
 # Database - PostgreSQL (SECRETS - keep in .env)
 # ─────────────────────────────────────────────
+# Main DB: SSO server reads + writes here.
 DB_HOST=localhost
 DB_PORT=5432
 DB_USER=postgres
@@ -18,9 +19,15 @@ DB_PASSWORD=postgres
 DB_NAME=sso
 # Connection string (optional: use instead of individual DB_HOST/DB_USER/DB_PASSWORD)
 DB_CONNECTION_STRING=
-# Read replica host (for read/write split)
+# Backup / HA DB: used for failover and (Phase 3) read-only fallback.
+# MultiPoolService auto-loads this when SSO_LOGIN_BACKUP_URL is set.
+SSO_LOGIN_BACKUP_URL=
+# Read-only fallback block (consumed by MultiPoolService.getReadPool()).
 DB_READ_HOST=
-DB_READ_PORT=5432
+DB_READ_PORT=5433
+DB_READ_USER=postgres
+DB_READ_PASSWORD=postgres
+DB_READ_NAME=sso_backup
 DB_POOL_MAX=50
 DB_POOL_MIN=5
 DB_POOL_ACQUIRE=30000