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>

src/lib/server/models/StockTransfer.ts

@@ -1,3 +1,4 @@
+import { tenantGuard } from "@/lib/server/tenantGuard";
 import { Schema, model, models, Model, Types, Document } from "mongoose";
 
 export interface IStockTransfer extends Document {
@@ -35,6 +36,8 @@ const stockTransferSchema = new Schema<IStockTransfer>(
 stockTransferSchema.index({ businessId: 1, clientUuid: 1 }, { unique: true });
 stockTransferSchema.index({ businessId: 1, createdAt: -1 });
 
+stockTransferSchema.plugin(tenantGuard);
+
 export const StockTransfer =
   (models.StockTransfer as Model<IStockTransfer>) ||
   model<IStockTransfer>("StockTransfer", stockTransferSchema);