feat(example-doc-vault): design document entity with state machine lifecycle

[crapougnax]

Part of parent story #36.

🎯 Goal

Implement the core domain entities and lifecycle state machine for the Document Vault reference application.

📋 Specification

1. Create Document model:
   • Properties: title (StringProperty), organizationId (StringProperty), state (StringProperty), fileUri (ObjectUri), financialAmount (NumberProperty).
   • Collection name: 'documents'.
2. Configure FSM lifecycle via @quatrain/state-machine:
   • States: DRAFT, IN_REVIEW, APPROVED, REJECTED, ARCHIVED.
   • Transition rules: Only author can submit (DRAFT ➔ IN_REVIEW), only approver can approve/reject.
3. Create AuditLog model for immutable transition history:
   • Collection name: 'audit_logs'.
   • Properties: targetUri (ObjectUri), action (StringProperty), actorId (StringProperty), timestamp (DateProperty).