Add SUPERUSERS support and ensure listed users are created/updated.

[vitali-yanushchyk-valor]

• Add environment variable SUPERUSERS.
• Builds a deduplicated, ordered list of superuser identities from env ADMIN_EMAIL (or --admin-email) plus env SUPERUSERS (or --superusers, space-separated).
• Each identity is treated as either an email (contains @) or a username; for usernames, a fallback email is derived as "{username}@{FALLBACK_EMAIL_DOMAIN}".
• Lookup semantics are split:
  • If identity is an email, it first searches by email (canonical), then by username == identity.
  • If identity is a username, it searches only by username (email is not used for lookup).
• If a user exists, it ensures is_staff=True and is_superuser=True (updates only when needed).
• If the user does not exist, it validates the email and creates the account via Django’s createsuperuser; password is only provided for ADMIN_EMAIL when ADMIN_PASSWORD is set. Otherwise creates the account with an unusable password.

AB#291835

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.45%. Comparing base (f2028eb) to head (cd2d592).
⚠️ Report is 4 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #252      +/-   ##
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+ Coverage    96.31%   96.45%   +0.13%     
===========================================
  Files           82       82              
  Lines         1955     1974      +19     
  Branches       117      118       +1     
===========================================
+ Hits          1883     1904      +21     
+ Misses          53       52       -1     
+ Partials        19       18       -1     

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