fix: Prevent duplicate subscription creation on admin register

- Check if subscription already exists before creating Tycoon subscription for admin users
- Fixes UNIQUE constraint failed: subscriptions.user_id error

backend/app/api/auth.py

@@ -106,8 +106,15 @@ async def register(
         user.is_verified = True  # Auto-verify admins
         await db.commit()
         
-        # Give admin Tycoon subscription
+        # Give admin Tycoon subscription (only if no subscription exists)
         from app.models.subscription import Subscription, SubscriptionTier, SubscriptionStatus, TIER_CONFIG
+        from sqlalchemy import select
+        
+        # Check if subscription already exists
+        existing_sub = await db.execute(
+            select(Subscription).where(Subscription.user_id == user.id)
+        )
+        if not existing_sub.scalar_one_or_none():
             tycoon_config = TIER_CONFIG.get(SubscriptionTier.TYCOON, {})
             subscription = Subscription(
                 user_id=user.id,