Fix month close migration constraints

backend/src/migrations.ts

@@ -7,9 +7,25 @@ async function findMigrationAdmin() {
 
 async function ensureClosureOwnership(table: string, column: string, primaryKeyColumns: string[], adminId: string | null) {
   await query(`ALTER TABLE ${table} ADD COLUMN IF NOT EXISTS user_id BIGINT REFERENCES users(id);`);
+
   if (adminId) {
     await query(`UPDATE ${table} SET user_id = $1 WHERE user_id IS NULL;`, [adminId]);
   }
+
+  const duplicateMatchers = [...primaryKeyColumns, "user_id"]
+    .map((key) => `older.${key} IS NOT DISTINCT FROM newer.${key}`)
+    .join(" AND ");
+
+  await query(`
+    DELETE FROM ${table} older
+    USING ${table} newer
+    WHERE ${duplicateMatchers}
+      AND (
+        older.closed_at < newer.closed_at
+        OR (older.closed_at = newer.closed_at AND older.ctid < newer.ctid)
+      );
+  `);
+
   await query(`
     DO $$
     BEGIN