fix: backfill missing api token beta values (#15405)

### What problem does this PR solve?

This PR updates `SystemService.ListAPITokens` to lazily backfill missing
`beta` values for API tokens, matching the Python behavior of
`/api/v1/system/tokens`.

### Type of change
  
  - When an API token has an empty `beta`, generate a new one.
  - Persist the generated `beta` back to the `api_token` table.
  - Keep the handler/routing unchanged.
- `GET /api/v1/system/tokens` now returns tokens with `beta` filled in
for older records that were missing it.
  - This aligns Go behavior with the Python implementation.