chore(auth): remove legacy rc_auth_uid/rc_uid/upsert_admin_user path; scope sign-in to ADMIN_ALLOWED_EMAILS; silence lockfile warning

Cleanup after Auth.js v5 became the only sign-in path. The platform
had three overlapping auth modes (dev cookie, legacy rc_auth_uid, Auth.js
JWT) and a pile of dead-code pages/routes that only existed to support
the legacy path.

What changed:

* getAdminUser() now has only two auth paths:
    1. dev_session cookie (auto-issued by src/proxy.ts for /admin/* when
       ALLOW_DEV_LOGIN is enabled)
    2. Auth.js v5 JWT (the encrypted cookie + auth() lookup)
  The legacy rc_auth_uid/rc_uid branch and the Supabase REST fetch
  against admin_users are gone.

* The signIn callback in src/lib/auth.ts enforces ADMIN_ALLOWED_EMAILS
  when set. Unset = open mode (backward compatible with demo/dev). Dev
  credentials provider is exempt. The new env var is wired through
  .env.example and .gitea/workflows/deploy.yml (read from
  secrets.ADMIN_ALLOWED_EMAILS, written to the server .env file).

* change-password/page.tsx now uses auth() server-side instead of
  fetching the deleted /api/auth/uid endpoint. The form is split into
  page.tsx (server component, auth check) + ChangePasswordForm.tsx
  (client component, form state). updatePasswordAction now reads the
  user id from auth() instead of the rc_auth_uid cookie.

* Deleted 14 dead-code files:
    - Pages: login2, logout, auth/callback, admin/debug-auth,
      admin/test-auth
    - API routes: api/login, api/logout, api/auth/uid, api/force-admin,
      api/set-auth-cookie, api/debug-cookie, api/debug-me,
      api/debug-auth
    - Actions: src/actions/login.ts
  These were the old email/password login, the old Supabase OAuth
  callback, the old /api/auth/uid probe, and a pile of debug endpoints
  that have been superseded by the new proxy + the new /login page.

* next.config.ts: set outputFileTracingRoot: '.' to silence the
  Next.js 16 lockfile-inference warning. Without this the build
  walked up from package.json looking for a lockfile, found the
  homelab runner's stale act cache at /home/tyler/.cache/act/.../package-lock.json,
  and warned on every build. '. resolves to the project root in both
  dev and CI, so it's the right answer.

Out of scope (deferred):

* src/actions/admin/users.ts still uses rc_auth_uid internally for its
  dev-bypass logic. It works (the rc_auth_uid branch is gated on
  NODE_ENV != 'production' and DEV_FORCE_UID), but it's now genuinely
  unreachable in production. Clean up in a follow-up.

Pre-flight:
* npx tsc --noEmit: clean
* npm run lint (touched files): clean
* npm run build: clean — proxy picked up, no lockfile warning, all
  93 static pages generated.

.env.example

@@ -40,6 +40,13 @@ GOOGLE_CLIENT_SECRET=
 # development. Default: enabled in dev only.
 ALLOW_DEV_LOGIN=true
 
+# Comma-separated list of email addresses allowed to sign in via Google
+# OAuth. If unset (or empty), any Google account can sign in and gets a
+# `platform_admin` row auto-created — fine for demo/dev. Set this in
+# production to lock sign-in down to a known set of admins.
+# Example: ADMIN_ALLOWED_EMAILS=tyler@example.com,sarah@example.com
+ADMIN_ALLOWED_EMAILS=
+
 # ── Supabase (legacy, being removed) ────────────────────────────────────────
 # Still used by the existing admin pages, server actions, and the
 # `getAdminUser` flow. Once the auth migration is complete and the