🔒 fix: Secure admin server actions by enforcing role checks

[marcin2121]

🎯 What: The vulnerability fixed
The server actions getAdminUsersDetails and getAdminUserCoupons in app/actions/get-admin-users.ts were completely unprotected. They bypassed Row-Level Security (RLS) and did not verify the caller's identity or role. Even worse, if the service role key was missing, it defaulted to the anonymous key.

⚠️ Risk: The potential impact if left unfixed
Any user, authenticated or not, could potentially invoke these server actions. This would allow an attacker to bypass RLS and read the entire list of users or read any user's private coupons from the database. This constitutes a severe Broken Access Control vulnerability.

🛡️ Solution: How the fix addresses the vulnerability

1. Removed the fallback from SUPABASE_SERVICE_ROLE_KEY to NEXT_PUBLIC_SUPABASE_ANON_KEY. If the admin key is missing, the action fails cleanly.
2. Initialized the server Supabase client (createServerClient) to fetch the current active session.
3. Explicitly checked that user.app_metadata?.role === 'admin' exists before executing the privileged supabaseAdmin code. This matches the security standard used in the middleware and guarantees that only verified administrators can call these functions.

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PR created automatically by Jules for task 12529834210628468742 started by @marcin2121

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