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/identity.test.ts

@@ -6,7 +6,7 @@ import * as login from "@/app/api/auth/login/route";
 import * as me from "@/app/api/auth/me/route";
 import * as platformBusinesses from "@/app/api/platform/businesses/route";
 import { ACCESS_COOKIE } from "@/lib/server/auth";
-import { json, makeTenant, req, uuid, type Actor, type Tenant } from "./helpers";
+import { json, makeTenant, req, uuid, type Actor, type Tenant, db } from "./helpers";
 
 const PERSON = { name: "Wanjiru", email: "wanjiru@aquaflow.test", phoneNumber: "0722000111" };
 const OPERATOR = "operator@aquaflow.test"; // listed in PLATFORM_ADMIN_EMAILS by tests/setup.ts
@@ -35,7 +35,7 @@ describe("email and phone are unique per business", () => {
     const b = await makeTenant("Beta");
     expect((await hire(a, a.owner)).status).toBe(201);
     expect((await hire(b, b.owner)).status).toBe(201);
-    expect(await User.countDocuments({ email: PERSON.email })).toBe(2);
+    expect(await db(() => User.countDocuments({ email: PERSON.email }))).toBe(2);
   });
 
   it("still refuses a duplicate inside the same business", async () => {