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>

src/app/api/auth/me/route.ts

@@ -4,7 +4,7 @@ import { User } from "@/lib/server/models/User";
 import { Branch } from "@/lib/server/models/Branch";
 import { Business } from "@/lib/server/models/Business";
 import { getAuth } from "@/lib/server/auth";
-import { isPlatformAdminEmail } from "@/lib/server/platform";
+import { isPlatformAdmin } from "@/lib/server/platform";
 import { isMpesaEnabled } from "@/lib/server/env";
 import { TIERS, effectiveMaxBranches, type TierId, type SubscriptionStatus } from "@/lib/tiers";
 
@@ -38,7 +38,7 @@ export async function GET(req: NextRequest) {
       branchName: branch?.branchName ?? null,
       phoneNumber: user.phoneNumber,
       email: user.email,
-      isPlatformAdmin: isPlatformAdminEmail(user.email),
+      isPlatformAdmin: isPlatformAdmin(user),
     },
     subscription: {
       tier,