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>

scripts/migrate-multitenant.ts

@@ -13,6 +13,7 @@ import { Expense } from "../src/lib/server/models/Expense";
 import { Purchase } from "../src/lib/server/models/Purchase";
 import { Business } from "../src/lib/server/models/Business";
 import { DEFAULT_TIER, TRIAL_DAYS } from "../src/lib/tiers";
+import { crossTenant } from "../src/lib/server/tenantGuard";
 
 async function migrate() {
   await connectDb();
@@ -75,7 +76,7 @@ async function migrate() {
   process.exit(0);
 }
 
-migrate().catch((err) => {
+crossTenant("one-off migration of data from before businesses existed", migrate).catch((err) => {
   console.error("migration failed:", err.message);
   process.exit(1);
 });

scripts/migrate-per-business-identity.ts

@@ -6,6 +6,7 @@ import mongoose from "mongoose";
 import { connectDb } from "../src/lib/server/db";
 import { User } from "../src/lib/server/models/User";
 import { platformAdminEmails } from "../src/lib/server/platform";
+import { crossTenant } from "../src/lib/server/tenantGuard";
 
 /**
  * One-off, safe to re-run. Moves user identity from platform-wide to
@@ -43,7 +44,7 @@ async function main() {
   await mongoose.disconnect();
 }
 
-main().catch((err) => {
+crossTenant("identity migration spans every business", main).catch((err) => {
   console.error("migration failed:", err.message);
   process.exit(1);
 });

scripts/seed.ts

@@ -14,6 +14,7 @@ import { User } from "../src/lib/server/models/User";
 import { Product } from "../src/lib/server/models/Product";
 import { Business } from "../src/lib/server/models/Business";
 import { isPlatformAdminEmail } from "../src/lib/server/platform";
+import { crossTenant } from "../src/lib/server/tenantGuard";
 
 /**
  * Run once against a fresh database: `npm run seed`.
@@ -107,7 +108,7 @@ async function seed() {
   process.exit(0);
 }
 
-seed().catch((err) => {
+crossTenant("seeding may find a legacy admin outside any business", seed).catch((err) => {
   console.error("seed failed:", err.message);
   process.exit(1);
 });