fix: make admin-user seeder idempotent

Skip inserting users that already exist in the database to prevent
duplicate key constraint violations on restarts.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/src/db/seeders/20200430130759-admin-user.ts

@@ -76,7 +76,17 @@ export default {
       },
     ];
 
-    await queryInterface.bulkInsert('users', rows);
+    // Check which users already exist to make seeder idempotent
+    const existing = await queryInterface.sequelize.query<{ id: string }>(
+      `SELECT id FROM users WHERE id IN (:ids)`,
+      { replacements: { ids }, type: 'SELECT' },
+    );
+    const existingIds = new Set(existing.map((row) => row.id));
+    const toInsert = rows.filter((row) => !existingIds.has(row.id as string));
+
+    if (toInsert.length > 0) {
+      await queryInterface.bulkInsert('users', toInsert);
+    }
   },
 
   down: async (queryInterface: QueryInterface) => {