Make email and phone unique per business, not platform-wide

One person could not work for two businesses, and signup and staff
creation told anyone whether an email or phone was registered anywhere
on the platform.

- User email and phone are now unique per business: indexes on
  (businessId, email) and (businessId, phoneNumber). Duplicate checks in
  signup and branch-manager creation are scoped to the business, so they
  no longer reveal other businesses' users.
- Login finds the email in every business and checks the password
  against each account. One match signs straight in; several return 300
  with the businesses to choose from, and the login page shows a picker.
  A wrong password returns the same 401 as before and reveals nothing;
  failures count against each candidate account, and a correct password
  only resets the accounts it opens.
- Platform-operator access now also needs a platformAdmin flag on the
  account. With per-business emails, an email match alone would have let
  any owner create a staff account with the operator's email and open the
  cross-tenant console. The flag is only set by the seed and the
  migration; removing the email from PLATFORM_ADMIN_EMAILS still revokes.
- npm run migrate:identity flags today's operators (while each listed
  email still matches one account) and syncs the User indexes, dropping
  the old unique email_1 and phoneNumber_1. Checked against a database in
  the old shape; safe to re-run.

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

.env.example

@@ -13,8 +13,10 @@ JWT_REFRESH_TTL=30d
 DELIVERY_MAX_DEVICES=2
 
 # ---- Platform operator ----
-# Comma-separated emails with cross-tenant access to /platform. Create these
-# accounts with `npm run seed`; public signup refuses these addresses.
+# Comma-separated emails with cross-tenant access to /platform. The account
+# must also carry the platformAdmin flag, set only by `npm run seed` or
+# `npm run migrate:identity`. Removing an email here revokes access.
+# Public signup refuses these addresses.
 PLATFORM_ADMIN_EMAILS=
 
 # ---- Invite links ----