Multi-user via PocketID: account linking, group gating, admin user management

PocketID OIDC already auto-provisioned users keyed by pocketid_sub, and the
data layer was already fully user-scoped. This adds the missing pieces for
running real multi-user:

- auth.py callback: link by email to an existing un-linked account (so the
  admin keeps their data when first signing in by passkey), collision-safe
  username generation, and request the `groups` scope.
- Group gating: optional pocketid_allowed_group (admin-config or
  POCKETID_ALLOWED_GROUP env); users lacking the group are rejected at the
  callback and redirected to /login?auth_error=not_authorized.
- New admin users API (app/api/users.py): list users, promote/demote admin
  (guards against demoting/locking out the last admin or yourself), and delete
  a user with ordered bulk deletes of all their data + on-disk files.
- ProfilePage: allowed-group field; LoginPage: rejected-login message;
  Layout: admin-only Users nav; new UsersPage.

Resync milevault_export to current source (it had drifted many features behind
— missing garmin_sync, npm-ci Dockerfile and @polyline-codec that broke its own
CI) and add POCKETID_ALLOWED_GROUP to .env.example.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

milevault_export/frontend/src/pages/LoginPage.jsx

@@ -7,7 +7,12 @@ import api from '../utils/api'
 export default function LoginPage() {
   const [username, setUsername] = useState('')
   const [password, setPassword] = useState('')
-  const [error, setError] = useState('')
+  const authError = new URLSearchParams(window.location.search).get('auth_error')
+  const [error, setError] = useState(
+    authError === 'not_authorized'
+      ? "Your account isn't permitted to access MileVault — ask the admin to add you to the allowed group."
+      : ''
+  )
   const { login, isLoading } = useAuthStore()
   const navigate = useNavigate()