bug: concurrent `/identity/connect/token` crashes Identity on premium license cache race

Steps To Reproduce

1. Run a self-hosted Bitwarden lite instance (v2026.8.1) with a premium user and a valid license at /etc/bitwarden/licenses/user/{selfHostedUserId}.json.
2. Log in on a mobile client (reproduced with Bitwarden iOS 2026.8.0) so the client holds a refresh token.
3. Trigger two overlapping POST /identity/connect/token requests for that user (refresh-token grant). This happens naturally when the iOS app retries / opens in the background; it can also be done with two parallel curls using the same refresh token.
4. Observe Identity logs and the client receiving HTTP 504 / an unexpected error.

Expected Result

Token issuance succeeds (or returns a normal OAuth error). Concurrent refresh requests for the same premium user must not crash Identity.

Actual Result

The first overlapping request can succeed or fail; the second throws an unhandled exception while building the access token. Identity then serves 504s until the process restarts.

System.ArgumentException: An item with the same key has already been added. Key: {selfHostedUserId}
   at System.Collections.Generic.Dictionary`2.Add(...)
   at Bit.Core.Billing.Services.LicensingService.ValidateUserPremiumAsync(User user)
      in /source/src/Core/Billing/Services/Implementations/LicensingService.cs:line 249
   at Bit.Identity.IdentityServer.ProfileService.GetProfileDataAsync(...)
   at Duende.IdentityServer.ResponseHandling.TokenResponseGenerator.ProcessRefreshTokenRequestAsync(...)

Follow-up requests then hit:

System.InvalidOperationException: Operations that change non-concurrent collections must have exclusive access. A concurrent update was performed on this collection and corrupted its state.
   at Bit.Core.Billing.Services.LicensingService.ValidateUserPremiumAsync(User user)

This is not a duplicate license file. There is a single user/{selfHostedUserId}.json. The dictionary key is the self-hosted user id; the Id inside the JSON is the cloud user id (expected).

Screenshots or Videos

No response

Additional Context

LicensingService keeps an in-memory cache:

private IDictionary<Guid, DateTime> _userCheckCache = new Dictionary<Guid, DateTime>();

In ValidateUserPremiumAsync (still present on master):

if (_userCheckCache.TryGetValue(user.Id, out var lastCheck))
{
    // ...
}
else
{
    _userCheckCache.Add(user.Id, now); // race
}

Two concurrent token requests for the same premium user both miss the cache and both call .Add. The unhandled exception aborts /identity/connect/token.

Suggested fix: use ConcurrentDictionary<Guid, DateTime> (or IMemoryCache) and TryAdd / indexer assignment instead of Dictionary.Add.

Related: https://github.com/bitwarden/server/issues/4274 (same stack traces; closed as an email/license mismatch; the race was not fixed).

Build Version

Bitwarden lite 2026.8.1 (ghcr.io/bitwarden/lite). Reproduced against server tag v2026.8.1; the racy _userCheckCache.Add is still on master.

Environment

Self-Hosted

Environment Details

• Deployment: Kubernetes, Bitwarden lite (single container)
• Identity listens on http://+:5005
• Premium individual license (not an organization)
• PostgreSQL 18
• ref #4274

Issue Tracking Info

• I understand that work is tracked outside of Github. A PR will be linked to this issue should one be opened to address it, but Bitwarden doesn't use fields like "assigned", "milestone", or "project" to track progress.