feat(example-doc-vault): implement dual-backend routing with automatic ObjectUri deduction

[crapougnax]

Part of parent story #36.

🎯 Goal

Configure a dual-database architecture demonstrating seamless multi-backend query resolution via ObjectUri.

📋 Specification

1. Register primary backend:
   • Backend.addBackend(postgresAdapter, 'postgres', true) (for active operational documents).
2. Register secondary archive backend:
   • Backend.addBackend(sqliteAdapter, 'archive', false) (for immutable audit logs and compliance records).
3. Demonstrate automatic ObjectUri routing without manual repository wiring:
   • Document.repository().read('doc-101') routes to postgres.
   • AuditLog.repository().read('archive:audit_logs/log-505') automatically routes to archive (SQLite).
4. Demonstrate cross-backend Active Record lifecycle (auditLog.save() writing automatically to archive).