feat(auth): add OIDC allowlist restrictions

[MauricioApu]

⚠️ Pre Checklist

• I have read through the Contributing Documentation.
• I have added relevant tests.
• I have added relevant documentation.
• I will add labels to the PR, such as pr-type/bug-fix, pr-type/feature-development, etc.

Summary

This PR introduces optional authorization filters to restrict OIDC logins by specific email addresses (OIDC_ALLOW_EMAILS) or domain suffixes (OIDC_ALLOW_DOMAINS).

• Default Unrestricted Behavior: If both configuration variables are empty or omitted, login access remains entirely unrestricted.
• Case-Insensitive & Formatted Matching: Email strings are properly trimmed of whitespace and converted to lower case to prevent user input discrepancies from blocking authentication.
• Granular Control: Administrators can now restrict access to specific developers or entire organizational domains seamlessly.

Does this close any open issues?

None. This is a straightforward enhancement implemented directly to address an internal deployment requirement.

Screenshots

Other Information

• Tests were added under backend/helpers/oidchelper/authorization_test.go confirming case-insensitivity, malformed string protection, domain parsing, and default pass-through state behaviors.
• Updated .env.example to document the new optional configuration keys.