Approval Workflow Engine with Delegation and Escalation

Administrator configures approval policies per document type (purchase orders, expenses, journal entries, payments) with threshold-based routing (e.g., invoices >$10,000 require CFO approval, <$10,000 require manager). When a document reaches "Pending Approval" state, the workflow engine creates approval tasks for the designated approvers and sends notifications. Approver reviews the document, adds comments, and approves or rejects. On rejection, the document returns to the requestor with comments. Approver can delegate to a named substitute for a date range;
if approval is not acted on within SLA, the system escalates to the next level and notifies the approver's manager.

approval_policy { policy_id: string, entity_id: string, document_type: string, threshold_minor: int64, currency_code: char(3), approver_role_id: string, escalation_hours: int, external_id: string };
approval_task { task_id: string, document_type: string, document_id: string, assignee_id: string, status: enum(PENDING,APPROVED,REJECTED,ESCALATED,DELEGATED), due_at: timestamp, external_id: string };
approval_audit { task_id: string, action: string, actor_id: string, comment: string, timestamp: timestamp };
(reference, product may differ).

POST /v1/approval-tasks/{id}/approve { comment } -> 200 { status: APPROVED };
POST /v1/approval-tasks/{id}/reject { comment } -> 200 { status: REJECTED };
POST /v1/approval-tasks/{id}/delegate { delegate_id, until_date } -> 200;
GET /v1/approval-tasks?assignee_id=&status=;
emits approval.task_created, approval.approved, approval.rejected, approval.escalated events;
idempotent via external_id.

PENDING -> APPROVED | REJECTED | ESCALATED;
delegated state DELEGATED -> PENDING(delegate);
terminal APPROVED or REJECTED;
guard: self-approval blocked;
SLA escalation after configurable hours;
approval history immutable.