Retroactively associate activity creators as participants

- Added a migration to backfill activity-user associations for creators.
- Introduced a temporary `by_migration` column for tracking and future cleanup.

src/Bundle/ChillActivityBundle/migrations/Version20251118124241.php

@@ -0,0 +1,50 @@
+<?php
+
+declare(strict_types=1);
+
+/*
+ * Chill is a software for social workers
+ *
+ * For the full copyright and license information, please view
+ * the LICENSE file that was distributed with this source code.
+ */
+
+namespace Chill\Migrations\Activity;
+
+use Doctrine\DBAL\Schema\Schema;
+use Doctrine\Migrations\AbstractMigration;
+
+/**
+ * Migration fixing the automatic association of users to activities (exchanges).
+ *
+ * Originally, the user who created an exchange was not automatically associated
+ * to it (the "TMS" column), which led to incomplete data and biased statistics.
+ *
+ * This migration:
+ *  - retroactively associates the creator of each exchange to the corresponding
+ *    activity;
+ *  - flags these backfilled associations with a temporary column so it is clear
+ *    they were added by this data correction and can be safely cleaned up later.
+ */
+final class Version20251118124241 extends AbstractMigration
+{
+    public function getDescription(): string
+    {
+        return 'Insert the creator of activity into the activity_user table';
+    }
+
+    public function up(Schema $schema): void
+    {
+        $this->addSql('ALTER TABLE activity_user ADD COLUMN by_migration BOOL DEFAULT FALSE');
+        $this->addSql("COMMENT ON COLUMN activity_user.by_migration IS 'For backup purpose - can be safely deleted after a while. See migration \\Chill\\Migrations\\Activity\\Version20251118124241'");
+
+        $this->addSql('INSERT INTO activity_user (activity_id, user_id, by_migration)
+            SELECT id, user_id, true FROM activity
+            ON CONFLICT DO NOTHING');
+    }
+
+    public function down(Schema $schema): void
+    {
+        $this->addSql('ALTER TABLE activity_user DROP COLUMN by_migration');
+    }
+}