fix: return 404 instead of 403 for cross-account tag access

get_tag now returns 404 for account-specific tags that belong to another
account, preventing resource existence confirmation.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/api/endpoints/tags.py

@@ -105,8 +105,8 @@ async def get_tag(
     # Check access: global tags visible to all, account tags only to account members
     if tag.account_id and tag.account_id != current_user.account_id and not current_user.is_super_admin:
         raise HTTPException(
-            status_code=status.HTTP_403_FORBIDDEN,
-            detail="You don't have access to this tag"
+            status_code=status.HTTP_404_NOT_FOUND,
+            detail="Tag not found"
         )
 
     return TagResponse.model_validate(tag)