CRASM-3710: Fix User Pending Bug - Dev/Staging

[aloftus23]

🗣 Description

Fixes users staying in the pending registration queue after a successful approve, including when org assignment already exists in the DB or register/approve was already completed.

Backend: add_user_to_org_v2 is idempotent (returns existing role on duplicate; IntegrityError fallback).
Frontend (RegionUsers.tsx): “Already approved” path completes org + invite_pending cleanup; 30s refresh of pending/member tables (paused during dialogs / hidden tab).

💭 Motivation and context

Staging logs showed: register/approve succeeding, then POST /v2/organizations/{id}/users returning 500 (duplicate role_user_id_organization_id), so update_user with invite_pending: false never ran. The UI could also treat “already approved” as done and remove the row without clearing pending. Stale pending rows (roles: [] after Manage Users) made the frontend retry org add and hit that 500.

🧪 Testing

Created backend and frontend unit tests.

✅ Pre-approval checklist

• This PR has an informative and human-readable title.
• Changes are limited to a single goal - eschew scope creep!
• All future TODOs are captured in issues, which are referenced in code comments.
• All relevant type-of-change labels have been added.
• I have read the CONTRIBUTING document.
• These code changes follow cisagov code standards.
• All relevant repo and/or project documentation has been updated to reflect the changes in this PR.
• Tests have been added and/or modified to cover the changes in this PR.
• All new and existing tests pass.
• Bump major, minor, patch, pre-release, and/or build versions as appropriate via the bump_version script if this repository is versioned and the changes in this PR warrant a version bump.
• Create a pre-release (necessary if and only if the pre-release version was bumped).

✅ Pre-merge checklist

• Revert dependencies to default branches.
• Finalize version.

✅ Post-merge checklist

• Create a release (necessary if and only if the version was bumped).