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

@@ -106,17 +106,24 @@ 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
-        tycoon_config = TIER_CONFIG.get(SubscriptionTier.TYCOON, {})
-        subscription = Subscription(
-            user_id=user.id,
-            tier=SubscriptionTier.TYCOON,
-            status=SubscriptionStatus.ACTIVE,
-            max_domains=tycoon_config.get("domain_limit", 500),
+        from sqlalchemy import select
+        
+        # Check if subscription already exists
+        existing_sub = await db.execute(
+            select(Subscription).where(Subscription.user_id == user.id)
         )
-        db.add(subscription)
-        await db.commit()
+        if not existing_sub.scalar_one_or_none():
+            tycoon_config = TIER_CONFIG.get(SubscriptionTier.TYCOON, {})
+            subscription = Subscription(
+                user_id=user.id,
+                tier=SubscriptionTier.TYCOON,
+                status=SubscriptionStatus.ACTIVE,
+                max_domains=tycoon_config.get("domain_limit", 500),
+            )
+            db.add(subscription)
+            await db.commit()
     
     # Generate verification token
     verification_token = secrets.token_urlsafe(32)