Add invoice edit/delete with EDI modification tracking

- PUT /invoices/:id now accepts lineItems, recalculates totals, and
  adjusts PO billed amounts when totals change
- Invoices edited after EDI submission get ediStatus "modified_after_send"
- DELETE relaxed from draft-only to any non-paid invoice
- Update README to reflect new endpoint behavior

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

README.md

@@ -32,9 +32,9 @@ API Gateway (HTTP API)
     │       ├── GET    /invoices             list
     │       ├── POST   /invoices             create (bills against PO)
     │       ├── GET    /invoices/:id         get one
-    │       ├── PUT    /invoices/:id         update
+    │       ├── PUT    /invoices/:id         update (full edit with line items; marks modified_after_send if already EDI-submitted)
     │       ├── PATCH  /invoices/:id/status  status transition
-    │       └── DELETE /invoices/:id         delete (drafts only)
+    │       └── DELETE /invoices/:id         delete (any non-paid invoice; reverses PO billed amount)
     │
     ├── /settings/**           ← Settings Lambda
     │       ├── GET  /settings              get user settings

lambdas/invoices/index.js

@@ -199,6 +199,59 @@ async function updateInvoice(id, event, user) {
     }
   });
 
+  // Handle line items update (recalculate totals)
+  if (body.lineItems && Array.isArray(body.lineItems) && body.lineItems.length > 0) {
+    const lineItems = body.lineItems.map((li, i) => {
+      if (!li.description) throw { statusCode: 400, message: `lineItems[${i}].description required` };
+      const qty       = parseFloat(li.qty || li.quantity || 1);
+      const unitPrice = parseFloat(li.unitPrice || li.price || 0);
+      return { description: li.description, qty, unitPrice, total: qty * unitPrice };
+    });
+
+    const subtotal = lineItems.reduce((s, li) => s + li.total, 0);
+    const taxRate  = parseFloat(body.taxRate ?? existing.Item.taxRate ?? 0);
+    const tax      = subtotal * (taxRate / 100);
+    const total    = subtotal + tax;
+
+    // If linked to a PO, check remaining balance (excluding current invoice's amount)
+    if (existing.Item.poId) {
+      const poResult = await db.send(new GetCommand({ TableName: POS_TABLE, Key: { id: existing.Item.poId } }));
+      if (poResult.Item) {
+        const billedWithoutThis = (poResult.Item.billed || 0) - existing.Item.total;
+        const remaining = poResult.Item.amount - billedWithoutThis;
+        if (total > remaining + 0.01) {
+          return badRequest(
+            `Invoice total ${fmt(total)} exceeds PO remaining balance ${fmt(remaining)} for ${poResult.Item.poNumber}`
+          );
+        }
+        // Update PO billed amount: remove old total, add new total
+        const diff = total - existing.Item.total;
+        if (Math.abs(diff) > 0.001) {
+          await db.send(new UpdateCommand({
+            TableName: POS_TABLE,
+            Key: { id: existing.Item.poId },
+            UpdateExpression: "SET billed = billed + :diff, updatedAt = :now",
+            ExpressionAttributeValues: { ":diff": diff, ":now": now },
+          }));
+        }
+      }
+    }
+
+    expressions.push("#lineItems = :lineItems", "#subtotal = :subtotal", "#taxRate = :taxRate", "#tax = :tax", "#total = :total");
+    names["#lineItems"] = "lineItems"; values[":lineItems"] = lineItems;
+    names["#subtotal"]  = "subtotal";  values[":subtotal"]  = subtotal;
+    names["#taxRate"]   = "taxRate";   values[":taxRate"]   = taxRate;
+    names["#tax"]       = "tax";       values[":tax"]       = tax;
+    names["#total"]     = "total";     values[":total"]     = total;
+  }
+
+  // If invoice was already submitted via EDI, mark as modified after send
+  if (existing.Item.ediStatus === "submitted") {
+    expressions.push("#ediStatus = :ediStatus");
+    names["#ediStatus"] = "ediStatus";
+    values[":ediStatus"] = "modified_after_send";
+  }
+
   const result = await db.send(new UpdateCommand({
     TableName: INV_TABLE,
     Key: { id },
@@ -263,8 +316,8 @@ async function deleteInvoice(id, user) {
   const db = getDocClient();
   const existing = await db.send(new GetCommand({ TableName: INV_TABLE, Key: { id } }));
   if (!existing.Item) return notFound("Invoice");
-  if (existing.Item.status !== "draft") {
+  if (existing.Item.status === "paid") {
-    return forbidden("Only draft invoices can be deleted. Cancel it first.");
+    return forbidden("Cannot delete a paid invoice");
   }
 
   await db.send(new DeleteCommand({ TableName: INV_TABLE, Key: { id } }));