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>

backend/app/models/user.py

@@ -33,6 +33,8 @@ class User(Base):
     pocketid_issuer = Column(String(512), nullable=True)
     pocketid_client_id = Column(String(256), nullable=True)
     pocketid_client_secret = Column(String(256), nullable=True)
+    # Only PocketID users in this group may sign in. Null/blank = allow all.
+    pocketid_allowed_group = Column(String(128), nullable=True)
 
     activities = relationship("Activity", back_populates="user", cascade="all, delete-orphan")
     health_metrics = relationship("HealthMetric", back_populates="user", cascade="all, delete-orphan")