feat: replace ID with UID

[sabhas]

BREAKING CHANGE: remove auto incremental ids from user, group and permissions and add a virtual uid property that returns string value of documents object id

Issue

Closes #361.

While addressing #359, the auto-incrementing sequence counter used for
User, Group, and Permission IDs was found to be broken on Cosmos DB
(it set every ID to 1). More generally, sequencing via a shared counter
document is not a good fit: it's a bottleneck on every
insert and a source of race conditions under concurrent writes.

Intent

Replace auto-incremental numeric IDs with a string uid across User,
Group, and Permission, and make the API consistently expose uid
instead of id wherever these entities appear in a response.

Implementation

• Added a virtual uid property to the User, Group, and Permission
  Mongoose models, returning this._id.toString(). No new field is stored
  and no data migration is required — every document already has _id,
  old and new alike.
• Removed the old sequencing mechanism entirely: the Counter model and
  getSequenceNextValue utility are gone, along with the bottleneck/race
  condition they caused.
• Updated all API responses that expose a user, group, or permission
  identifier — including login (POST /SASLogon/login), session
  (GET /SASjsApi/session), and the user/group/permission endpoints
  — to consistently return uid.
• Updated the web frontend (permission management, user profile, session
  restoration on page load) to read uid instead of id.

This is a breaking change for any existing client relying on numeric
IDs from this API — endpoints now return an opaque string identifier
instead.

Checks

• Code is formatted correctly (npm run lint:fix).
• Any new functionality has been unit tested.
• All unit tests are passing (npm test).
• All CI checks are green.
• Reviewer is assigned.

[YuryShkoda]

see comments above