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>

frontend/src/App.jsx

@@ -12,6 +12,7 @@ import SegmentsPage from './pages/SegmentsPage'
 import RecordsPage from './pages/RecordsPage'
 import UploadPage from './pages/UploadPage'
 import ProfilePage from './pages/ProfilePage'
+import UsersPage from './pages/UsersPage'
 
 function RequireAuth({ children }) {
   const token = useAuthStore((s) => s.token)
@@ -39,6 +40,7 @@ export default function App() {
         <Route path="records" element={<RecordsPage />} />
         <Route path="upload" element={<UploadPage />} />
         <Route path="profile" element={<ProfilePage />} />
+        <Route path="users" element={<UsersPage />} />
       </Route>
     </Routes>
   )