Enforce tenant isolation in the data layer instead of per route

Every route had to remember to add the business filter to every query;
nothing stopped one that forgot. The audit for this change found several
that had: the M-Pesa status re-check, the stock ledger's duplicate checks,
the daily-inventory and branch-price upserts, a cash-sale replay lookup,
several load-by-id lookups, and every populate().

- tenantGuard: a Mongoose plugin on all ten business-scoped models. Any
  find/update/delete/count/distinct, or an aggregate whose first stage is
  not a $match, that does not pin exactly one businessId throws
  TenantScopeError before reaching the database ($in, $exists and other
  operators do not count).
- crossTenant("reason", fn): the explicit, documented exemption for the
  queries that must span businesses - login's email lookup, refresh and
  invite-token lookups, the M-Pesa callback, the platform console and the
  maintenance scripts. git grep crossTenant lists them all.
- scopedPopulate: populated branch, product and staff names are also
  restricted to the business.
- Every flagged query site now names the business (or is exempted with a
  reason).
- Tests: a check that every model with a businessId field carries the
  guard; the guard refusing unscoped queries of every kind; and a route
  sweep where business A sends business B's ids to every read route
  (nothing of B's comes back) and every write route (all refused, B's
  data unchanged). Test-side database inspections go through db(), an
  explicit test exemption. Removing one route's business filter makes the
  suite fail.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/helpers.ts

@@ -7,6 +7,7 @@ import { User, UserRole } from "@/lib/server/models/User";
 import { Product } from "@/lib/server/models/Product";
 import { signAccessToken } from "@/lib/server/jwt";
 import { ACCESS_COOKIE } from "@/lib/server/auth";
+import { crossTenant } from "@/lib/server/tenantGuard";
 
 let phoneSeq = 0;
 function nextPhone(): string {
@@ -131,3 +132,8 @@ export async function json<T = any>(res: Response): Promise<{ status: number; bo
 export function uuid(): string {
   return randomUUID();
 }
+
+/** Tests may inspect and arrange any business's data directly. */
+export function db<T>(fn: () => PromiseLike<T>): Promise<T> {
+  return crossTenant("test inspects the database directly", fn);
+}