fix(pos): fix uncaught 500 on checkout, allow overselling into negative stock

- checkoutSale's stock-availability loop called pb.getOne() with no
  try/catch, so a stale/deleted stock_item_id in a persisted cart
  threw all the way up to a bare 500 with no useful message. Now
  caught and returned as a clear pb_error the till can display.
- Removed the insufficient-stock block entirely: the till is used
  ahead of a physical stock take, so a sale must never be blocked
  for lack of stock — current_qty is now allowed to go negative and
  gets corrected at the next stock take. Drops the now-dead
  insufficient_stock/shortages branches from the route, service
  types, and client error handling.

public/pos/pos-app.js

@@ -884,18 +884,15 @@ async function confirmSale(){
       sale.synced = true;
       sale.serverRef = j.sale && j.sale.id;
     } else {
-      // Try to read the body so we can tell apart stock-shortage
-      // (409) from generic 4xx/5xx. The route returns JSON on every
-      // non-2xx path, so this should always parse.
+      // The route returns JSON on every non-2xx path, so this should
+      // always parse. Overselling is allowed on purpose (no stock
+      // check blocks the sale) — every rejection here is a real
+      // validation/server failure, not a stock shortage.
       let body = null;
       try { body = await r.json(); } catch (_) { /* not json */ }
-      if (body && body.reason === 'insufficient_stock' && Array.isArray(body.shortages) && body.shortages.length) {
-        serverError = { kind: 'insufficient_stock', shortages: body.shortages };
-      } else {
-        const msg = body && body.message ? body.message : ('server returned ' + r.status);
-        serverError = { kind: 'other', message: msg };
-        throw new Error(msg);
-      }
+      const msg = body && body.message ? body.message : ('server returned ' + r.status);
+      serverError = { message: msg };
+      throw new Error(msg);
     }
   }catch(e){
     console.warn('Sale queued for sync', e);
@@ -917,18 +914,7 @@ async function confirmSale(){
   // cart (remove a line, lower the qty) and try again. The cart
   // stays open.
   if (serverError) {
-    if (serverError.kind === 'insufficient_stock') {
-      const lines = serverError.shortages.map(s => {
-        const item = state.cart.get(s.stock_item_id);
-        const name = item && item.p
-          ? (item.p.description || item.p.sku || s.stock_item_id)
-          : s.stock_item_id;
-        return `${name}: only ${s.available} in stock (you asked for ${s.requested})`;
-      });
-      showToast('Not enough stock — ' + lines.join('; '));
-    } else {
-      showToast('Sale rejected: ' + (serverError.kind === 'other' ? serverError.message : 'Unknown error'));
-    }
+    showToast('Sale rejected: ' + serverError.message);
     if (btn) {
       btn.disabled = false;
       btn.innerHTML = '<svg viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2"><path d="M5 12l5 5 9-11"/></svg>Complete sale';

public/pos/sw.js

@@ -1,5 +1,5 @@
 // Service worker — cache app shell, network-first for HTML + theme.js, cache-first for other static.
-const CACHE = "pos-v21";
+const CACHE = "pos-v22";
 const APP_SHELL = [
   './',
   './index.html',

src/app/api/v1/pos/checkout/route.test.ts

@@ -52,12 +52,11 @@ describe("POST /api/v1/pos/checkout", () => {
     expect(body.sale.id).toBe("sale-1");
   });
 
-  it("returns 409 when stock is insufficient", async () => {
+  it("returns 400 when a stock item can't be found (e.g. deleted since being added to the cart)", async () => {
     vi.mocked(checkoutSale).mockResolvedValue({
       ok: false,
-      reason: "insufficient_stock",
-      message: "one or more items do not have enough stock",
-      shortages: [{ stock_item_id: "si1", requested: 5, available: 2 }],
+      reason: "pb_error",
+      message: "Stock item si1 could not be found (it may have been deleted) — remove it from the cart and try again: not found",
     });
     const res = await POST(
       checkoutRequest({
@@ -65,7 +64,7 @@ describe("POST /api/v1/pos/checkout", () => {
         payment_method: "cash",
       }),
     );
-    expect(res.status).toBe(409);
+    expect(res.status).toBe(400);
   });
 
   it("returns 400 for other checkout failures", async () => {

src/app/api/v1/pos/checkout/route.ts

@@ -33,8 +33,12 @@ import {
  * on success (stockErrors non-empty means the sale was recorded but one
  * or more lines didn't decrement stock — needs manual reconciliation;
  * duplicate true means this was a replay of an existing sale),
- * 409 when stock is insufficient, 400 for other validation failures,
- * 401 for a missing/wrong POS_API_KEY.
+ * 400 for validation failures, 401 for a missing/wrong POS_API_KEY.
+ *
+ * Overselling is allowed on purpose: the till is used ahead of a
+ * physical stock take, so a sale is never blocked for insufficient
+ * stock — `stock_items.current_qty` is allowed to go negative and
+ * gets corrected at the next stock take.
  */
 export async function POST(req: NextRequest) {
   try {
@@ -74,8 +78,7 @@ export async function POST(req: NextRequest) {
   });
 
   if (!result.ok) {
-    const status = result.reason === "insufficient_stock" ? 409 : 400;
-    return NextResponse.json(result, { status });
+    return NextResponse.json(result, { status: 400 });
   }
   return NextResponse.json(result);
 }

src/lib/server/pos-service.ts

@@ -208,9 +208,8 @@ export type PosCheckoutResult =
     }
   | {
       ok: false;
-      reason: "no_items" | "invalid_line" | "insufficient_stock" | "pb_error";
+      reason: "no_items" | "invalid_line" | "pb_error";
       message: string;
-      shortages?: Array<{ stock_item_id: string; requested: number; available: number }>;
     };
 
 /** PB filter strings are built by hand here, so quote-escape anything user-supplied. */
@@ -299,24 +298,30 @@ export async function checkoutSale(
     }
   }
 
-  // 1. Check availability for every line before writing anything.
-  const shortages: Array<{ stock_item_id: string; requested: number; available: number }> = [];
+  // 1. Fetch every line's stock item. Overselling is allowed on
+  // purpose — the till gets used ahead of a physical stock take, so
+  // current_qty is allowed to go negative rather than blocking the
+  // sale; it gets corrected at the next stock take. We still need
+  // each item's row (for unit_cost and the qty decrement below), and
+  // a cart can outlive the item it references (persisted client-side,
+  // then the stock item is deleted/deactivated before checkout) — an
+  // unhandled getOne() failure here used to propagate all the way up
+  // to a bare 500 with no useful message. Catch it and report which
+  // line is the problem instead.
   const stockItems = new Map<string, StockItem>();
   for (const line of input.items) {
-    const item = (await pb.collection("stock_items").getOne(line.stock_item_id)) as StockItem;
-    stockItems.set(line.stock_item_id, item);
-    const available = Number(item.current_qty ?? 0);
-    if (available < line.qty) {
-      shortages.push({ stock_item_id: line.stock_item_id, requested: line.qty, available });
+    let item: StockItem;
+    try {
+      item = (await pb.collection("stock_items").getOne(line.stock_item_id)) as StockItem;
+    } catch (err) {
+      const message = err instanceof Error ? err.message : String(err);
+      return {
+        ok: false,
+        reason: "pb_error",
+        message: `Stock item ${line.stock_item_id} could not be found (it may have been deleted) — remove it from the cart and try again: ${message}`,
+      };
     }
-  }
-  if (shortages.length > 0) {
-    return {
-      ok: false,
-      reason: "insufficient_stock",
-      message: "one or more items do not have enough stock",
-      shortages,
-    };
+    stockItems.set(line.stock_item_id, item);
   }
 
   // 2. Totals — VAT-inclusive shelf pricing (matches the existing POS