Persist event registration slugs in session

[maebeale]

What is the goal of this PR and why is this important?

• Public (non-authenticated) users who register for events lose the ?reg= query param when navigating away from the event show page
• This causes the "View Registration" button to disappear on the events index and on revisits to the event show page
• Logged-in users aren't affected since their registration is looked up via current_user.person

How did you approach the change?

• Added EventRegistrationSession concern that stores registration slugs in session[:event_reg_slugs] as a { event_id => slug } hash
• Slugs are stored on registration create, ticket view, and reactivation; cleared on cancellation
• _registration_section.html.erb and _card.html.erb fall back to session when params[:reg] is absent
• "Back to Events" navigation links pass ?reg= as belt-and-suspenders (works even without a session, e.g. shared URLs)
• Stale slugs are handled gracefully — find_by returns nil → active? is false → user sees "Register" instead

UI Testing Checklist

• Register as public user → visit ticket → go to events index → "View registration" button appears on card
• Click into event show from index → "View Registration" button still visible
• Cancel registration → "View Registration" disappears from both index and show
• Reactivate registration → "View Registration" reappears
• Logged-in user registration flow still works as before

Anything else to add?

• Session size is negligible (~30 bytes per registration)
• No N+1 concern on index — session lookup is in-memory, DB query only fires for events with a stored slug

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