Know which overdue customers a reminder email will actually reach

**Objective:** For every Sage Intacct customer with an invoice at least `{min-days-overdue}` days past due (default 30), work out whether a collections email would reach them, using their Constant Contact status.

**Step 1: Find overdue invoices**

Call `intacct-get-invoice` with `due-date.to` set to today minus the overdue days and `total-due.min` set to 0.01. Group the rows by customer-id, keeping the total due and the oldest due date. If `{min-total-due}` is given, drop customers whose overdue total is below it. If nothing is overdue, say so and stop.

**Step 2: Get each customer's email**

For each customer-id, call `intacct-get-customer` with `customer-id`. Keep email and customer-name. A customer with a blank email goes straight to the No email on file group.

**Step 3: Carry the email across**

For each email, call `constant-contact-get-contact` with `email-address`. Then:

* not found: Not in Constant Contact
* permission-to-send is unsubscribed: Unsubscribed
* otherwise call `constant-contact-get-campaign-bounce` with `contact-id`; any bounce in the last 180 days: Bouncing
* otherwise call `constant-contact-get-campaign-open` with `contact-id` and `created-date.from` set to today minus 90 days; at least one open: Reachable and engaged; none: Reachable, not opening

**Step 4: Report**

One table sorted by overdue total, largest first: customer name, customer-id, email, overdue total, oldest due date, reachability group. Then a count per group, and a one-line recommendation for the groups that need another channel (phone, letter) rather than email.

**Rules & Guidelines:**

* Tools whose names start with intacct are on the intacct server; tools whose names start with constant-contact are on the constant-contact server.
* Read-only. Do not call `intacct-update-customer`, `constant-contact-update-contact`, `constant-contact-add-contact` or `constant-contact-add-campaign`. Do not send anything.
* Unsubscribed means marketing consent was withdrawn. Say that a transactional collections notice may still be allowed, but do not decide that for the user.

**Validation:**

* If more than 100 customers are overdue, process the 100 with the largest overdue totals and say how many were left out.
* Compare emails case-insensitively and trimmed. If a customer's email field holds several addresses, check each one and use the best result.