Fix AttributeError: can't set attribute 'is_active' in User model

Flask-Login's UserMixin provides is_active as a read-only property.
Attempting to set it as an instance attribute caused a conflict.

Solution:
- Store active status in private attribute _is_active
- Override is_active property to return custom value
- Update to_dict() to use _is_active

This allows proper Flask-Login integration while maintaining
custom active status tracking.

Bug found during login testing: AttributeError when calling
User.get_by_username() which triggered from_dict() → __init__().

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

app/models/user.py

@@ -25,12 +25,17 @@ class User(UserMixin):
         self.password_hash = password_hash
         self.role = role
         self.product_ids = product_ids or []
-        self.is_active = is_active
+        self._is_active = is_active
 
     def get_id(self):
         """Get user ID for Flask-Login"""
         return self.user_id
 
+    @property
+    def is_active(self):
+        """Check if user account is active (Flask-Login property)"""
+        return self._is_active
+
     @property
     def is_authenticated(self):
         """Check if user is authenticated"""
@@ -78,7 +83,7 @@ class User(UserMixin):
             'password_hash': self.password_hash,
             'role': self.role,
             'product_ids': self.product_ids,
-            'is_active': self.is_active
+            'is_active': self._is_active
         }
 
     @classmethod