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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>