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/concurrency.test.ts

@@ -4,7 +4,7 @@ import { StockTransfer } from "@/lib/server/models/StockTransfer";
 import { StockLock } from "@/lib/server/models/StockLock";
 import * as adjustments from "@/app/api/stock/adjustments/route";
 import * as transfers from "@/app/api/stock/transfers/route";
-import { makeTenant, req, uuid, type Tenant } from "./helpers";
+import { makeTenant, req, uuid, type Tenant, db } from "./helpers";
 
 async function openingStock(t: Tenant, quantity: number) {
   await adjustments.POST(
@@ -35,7 +35,7 @@ describe("concurrent stock changes", () => {
     expect(statuses.filter((s) => s === 409)).toHaveLength(3);
     expect(await stockOf(t.businessId, t.branchA, t.water20)).toBe(2);
     expect(await stockOf(t.businessId, t.branchB, t.water20)).toBe(8);
-    expect(await StockTransfer.countDocuments({})).toBe(2);
+    expect(await db(() => StockTransfer.countDocuments({}))).toBe(2);
   });
 
   it("holds across repeated races, not just once", async () => {