feat(statements): add overdue-customers query API

Add ns_app/api/statements.py with get_customers_with_overdue_invoices
(one aggregated row per customer with overdue Sales Invoices) plus the
_get_outstanding_invoices / _aging_bucket helpers used to build the
per-customer statement.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

ns_app/api/statements.py

@@ -0,0 +1,107 @@
+"""Customer account statements.
+
+Generates printable, one-customer-per-page account statements for customers with
+overdue invoices, formatted for a standard double-window envelope. Statement
+generation also books a late-payment fee to the ledger (see the late-fee helpers
+added alongside the generator).
+"""
+
+import frappe
+from frappe import _
+from frappe.utils import getdate, nowdate
+
+# Roles allowed to run collections/statement actions.
+ALLOWED_ROLES = [
+    "System Manager",
+    "Sales User",
+    "Sales Manager",
+    "Accounts User",
+    "Accounts Manager",
+]
+
+
+@frappe.whitelist()
+def get_customers_with_overdue_invoices():
+    """Return one row per customer that has at least one overdue Sales Invoice.
+
+    A Sales Invoice is overdue when it is submitted, still has an outstanding
+    balance, and its due date is in the past.
+    """
+    frappe.only_for(ALLOWED_ROLES)
+
+    today = nowdate()
+    rows = frappe.get_all(
+        "Sales Invoice",
+        filters={
+            "docstatus": 1,
+            "outstanding_amount": [">", 0],
+            "due_date": ["<", today],
+        },
+        fields=[
+            "customer",
+            "customer_name",
+            "count(name) as overdue_count",
+            "sum(outstanding_amount) as total_outstanding",
+            "min(due_date) as oldest_due_date",
+        ],
+        group_by="customer, customer_name",
+        order_by="total_outstanding desc",
+    )
+
+    for row in rows:
+        row["max_days_overdue"] = (
+            (getdate(today) - getdate(row.oldest_due_date)).days
+            if row.oldest_due_date
+            else 0
+        )
+
+    return rows
+
+
+def _aging_bucket(days_overdue):
+    """Map days-overdue to a standard aging bucket label."""
+    if days_overdue <= 0:
+        return "Current"
+    if days_overdue <= 30:
+        return "1-30"
+    if days_overdue <= 60:
+        return "31-60"
+    if days_overdue <= 90:
+        return "61-90"
+    return "90+"
+
+
+def _get_outstanding_invoices(customer):
+    """Return all open (submitted, unpaid) Sales Invoices for a customer.
+
+    The statement lists the full open balance, so this includes not-yet-due
+    invoices; each row is annotated with days overdue, an overdue flag, and its
+    aging bucket.
+    """
+    today = getdate(nowdate())
+    invoices = frappe.get_all(
+        "Sales Invoice",
+        filters={
+            "customer": customer,
+            "docstatus": 1,
+            "outstanding_amount": [">", 0],
+        },
+        fields=[
+            "name",
+            "posting_date",
+            "due_date",
+            "outstanding_amount",
+            "grand_total",
+            "company",
+        ],
+        order_by="due_date asc",
+    )
+
+    for inv in invoices:
+        due = getdate(inv.due_date) if inv.due_date else None
+        days = (today - due).days if due else 0
+        inv["days_overdue"] = days if days > 0 else 0
+        inv["is_overdue"] = days > 0
+        inv["aging_bucket"] = _aging_bucket(inv["days_overdue"])
+
+    return invoices