Investigate missing foreign keys on `messagecategory` table.

Currently the table looks like this:

CREATE TABLE `messagecategory` (
  `created_at` datetime NOT NULL DEFAULT CURRENT_TIMESTAMP,
  `id` bigint(20) NOT NULL AUTO_INCREMENT,
  `message_id` bigint(20) NOT NULL,
  `category_id` bigint(20) NOT NULL,
  PRIMARY KEY (`id`),
  KEY `ix_messagecategory_category_id` (`category_id`),
  KEY `ix_messagecategory_created_at` (`created_at`),
  KEY `message_category_ids` (`message_id`,`category_id`)
);

Notice that there are no foreign key constraints on message_id and category_id columns. This can lead to to data corruption and exceptions on Python side. We should investigate if they were omitted on purpose. If not we should add them.

Start by locating the schema or migration that creates the messagecategory table and inspect the related message and category tables. Determine whether the missing constraints are intentional and check existing tests or migration practices for relationship integrity. Done means the decision is documented and, if appropriate, the constraints are added without breaking existing data.