Run Automated Dunning / Collections Workflow

Nightly collections engine scans all open invoices exceeding their due date;
invoices are assigned to a dunning level based on days-overdue thresholds configured per customer segment (e.g., Level 1 at 7 days, Level 2 at 30 days, Level 3 at 60 days);
for each level transition the system generates a dunning email from a configurable template and sends it to the customer's billing contact;
a collections case is opened or escalated in the system;
AR clerk receives a daily collections task list;
customer responses (promise-to-pay, dispute) are logged against the case;
on receipt the invoice exits the dunning queue.

dunning_policy { id: string, customer_segment: string, levels: [{level: int, days_overdue: int, template_id: string}] };
collections_case { id: string, invoice_id: invoice_*, customer_id: cust_*, dunning_level: int, opened_at: timestamp, status: enum, promise_to_pay_date: date };
dunning_event { case_id: string, level: int, sent_at: timestamp, delivery_status: enum };
(reference, product may differ).

POST /v1/dunning/run { entity_id, as_of_date } -> 202 { run_id, invoices_processed: int };
GET /v1/collections-cases?customer_id=&status=;
POST /v1/collections-cases/{id}/promise-to-pay { pay_date } -> 200;
emits ar.dunning.sent per level transition;
idempotent re-run produces no duplicate emails.

OPEN -> LEVEL_1 -> LEVEL_2 -> LEVEL_3 -> ESCALATED;
terminal CLOSED;
guard: dispute hold and payment plan hold pause all level transitions;
VIP hold blocks entry to dunning.