fix(auth): add username + passwordHash fields to users collection

The production UsersRepository reads and writes username + passwordHash
via the Payload local API, but the users collection never declared them,
so production sign-up/sign-in was broken (audit finding B1). passwordHash
uses access.read: () => false so credential material never serializes
through any Payload API surface; the repository still reads it with
overrideAccess: true. A contract-shaped test pins every repo-used field
(USERS_REPOSITORY_FIELDS) against the collection config so drift fails
at test time without a database.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

packages/auth/src/integrations/cms/collections/users.ts

@@ -18,6 +18,35 @@ export const users: CollectionConfig = {
     subject: { kind: "self", field: "id" },
   },
   fields: [
+    {
+      // Read/written by the production UsersRepository (getUserByUsername,
+      // createUser). Pinned by collections/users.test.ts against
+      // USERS_REPOSITORY_FIELDS so repo <-> collection drift fails fast.
+      name: "username",
+      type: "text",
+      required: true,
+      unique: true,
+      index: true,
+      custom: {
+        pii: {
+          category: "identification-username",
+          purpose: ["service-delivery"],
+          exportable: true,
+          restrictable: true,
+        },
+      },
+    },
+    {
+      // Credential material — must never leave the server. `access.read`
+      // returns false unconditionally so the field is stripped from every
+      // REST/GraphQL/admin API response; the auth repository still reads it
+      // through the local API with `overrideAccess: true`.
+      name: "passwordHash",
+      type: "text",
+      required: true,
+      admin: { hidden: true },
+      access: { read: () => false },
+    },
     {
       name: "displayName",
       type: "text",