Fix ScenarioUser coupling bug

[louispt1]

This PR:

• Fixed bug in ScenarioUser.couple_existing_user which was looking up by user_id rather than user_email
• Expanded test coverage: added spec covering the user coupling logic more fulsomely

Motivation:

Users added as collaborators via MyETM were not being properly linked to their accounts in ETEngine. The couple_existing_user hook never actually coupled users because it looked up User.find_by(id: user_id)
when user_id was nil. This meant:

• Existing users added by email weren't immediately coupled to their account
• Collaboration permissions weren't working across MyETM and ETEngine

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Testing

Added spec to check:

• Existing users are automatically coupled when added by email
• Non-existent users remain pending with user_email set
• Duplicate user detection works correctly via unique indices
• User signup triggers automatic coupling of pending ScenarioUser records

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Open questions:

• Do we want to show pending users (as in, scenario user with email but no id) on the etengine inspect scenario view?

Goes with this MyETM PR

[noracato]

Nice catch!