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/Dockerfile

@@ -1,8 +1,8 @@
 FROM node:20-alpine AS builder
 
 WORKDIR /app
-COPY package*.json ./
-RUN npm ci
+COPY package.json ./
+RUN npm install
 
 COPY . .
 ARG VITE_API_URL=/api
@@ -15,4 +15,4 @@ RUN npm run build
 FROM nginx:alpine
 COPY --from=builder /app/dist /usr/share/nginx/html
 COPY nginx-spa.conf /etc/nginx/conf.d/default.conf
-EXPOSE 80
+EXPOSE 80