DPO and payment SLA by supplier

# DPO and supplier payment terms

## Purpose
Calculate the Days Payable Outstanding (DPO) and analyze actual payment terms per supplier. Identify suppliers systematically paid late (relationship risk) or too early (cash management not optimized).

**Limitation:** without contractual payment terms in Spendesk (e.g. net 30, net 60), comparison against contractual terms is not possible automatically.

## Instructions

### Step 1 — Retrieve payables paid during the period
```
Call get_payables with:
  - paymentStatus: paid (or exported in ERPs with no paymentDate)
  - Period: last 3 or 6 months
  - Retrieve: id, supplierId, amount, currency, invoiceDate, paymentDate (or exportedAt)
```

### Step 2 — Calculate the payment delay per invoice
```
For each payable with invoiceDate and paymentDate:
  delayDays = paymentDate - invoiceDate

Exclude outliers > 365 days (data errors) and < 0 days (prepayments).
```

### Step 3 — Overall DPO calculation
```
Method 1 (simple):
  DPO = Average of delayDays across all paid invoices

Method 2 (amount-weighted):
  Weighted_DPO = Sum(delayDays × amount) / Sum(amount)

Present both for comparison.
```

### Step 4 — Per-supplier analysis (top 20)
```
For the 20 suppliers with the highest volume of paid invoices:
  Supplier_DPO = average delayDays for that supplier
  Categories:
    < 15 days: very fast payment (cash management not optimized?)
    15-30 days: standard payment
    30-45 days: average delay
    > 45 days: late payment (relationship risk)
```

### Step 5 — DPO report
```
Present:
1. Overall simple and weighted DPO
2. Distribution: % of invoices paid < 15 days | 15-30 days | 30-45 days | > 45 days
3. Top 20 suppliers table: Name | Average DPO | Nb invoices | Total amount
4. Alerts:
   🔴 Suppliers with DPO > 60 days (dispute risk)
   🟡 Suppliers with DPO > 45 days (to monitor)
   💡 Suppliers paid < 15 days with high volume (optimization possible)
```

## Points to watch
- The `paymentDate` field may be missing — use `exportedAt` as a proxy (ERP export date ≈ payment date)
- Expense claims have different delays from supplier invoices — treat them separately
- The optimal DPO depends on contractual terms — this skill provides the indicator, not the benchmark

## Example prompts
- "What is our average DPO over the last 6 months?"
- "Which suppliers wait the longest to be paid?"
- "Show me payment delays by supplier"