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/api/profile.py

@@ -121,6 +121,7 @@ class PocketIDConfig(BaseModel):
     issuer: Optional[str] = None
     client_id: Optional[str] = None
     client_secret: Optional[str] = None
+    allowed_group: Optional[str] = None
 
 
 @router.get("/pocketid-config")
@@ -131,10 +132,12 @@ async def get_pocketid_config(current_user: User = Depends(get_current_user)):
     # Show DB config if set, fall back to env
     issuer = current_user.pocketid_issuer or settings.pocketid_issuer
     client_id = current_user.pocketid_client_id or settings.pocketid_client_id
+    allowed_group = current_user.pocketid_allowed_group or settings.pocketid_allowed_group
     return {
         "issuer": issuer or "",
         "client_id": client_id or "",
         "client_secret_set": bool(current_user.pocketid_client_secret or settings.pocketid_client_secret),
+        "allowed_group": allowed_group or "",
         "enabled": bool(issuer and client_id),
     }
 
@@ -153,6 +156,8 @@ async def save_pocketid_config(
         current_user.pocketid_client_id = body.client_id or None
     if body.client_secret is not None:
         current_user.pocketid_client_secret = body.client_secret or None
+    if body.allowed_group is not None:
+        current_user.pocketid_allowed_group = body.allowed_group.strip() or None
     await db.commit()
     return {"status": "ok"}