fix: do not allow creating cr for same environment

src/migrations/20250515252526-cr-uniqueness.js

@@ -0,0 +1,32 @@
+exports.up = (db, callback) => {
+    db.runSql(
+        `
+            WITH ranked AS (
+                SELECT id,
+                       ROW_NUMBER() OVER (
+                       PARTITION BY created_by, project, environment
+                       ORDER BY created_at DESC
+                   ) AS rn
+                FROM change_requests
+                WHERE state NOT IN ('Applied', 'Cancelled', 'Rejected', 'Scheduled')
+            )
+            UPDATE change_requests
+            SET state = 'Cancelled'
+            WHERE id IN (
+                SELECT id FROM ranked WHERE rn > 1
+            );
+
+            CREATE UNIQUE INDEX unique_pending_request_per_user_project_env
+                ON change_requests (created_by, project, environment)
+                WHERE state NOT IN ('Applied', 'Cancelled', 'Rejected', 'Scheduled');
+        `,
+        callback,
+    );
+};
+
+exports.down = (db, callback) => {
+    db.runSql(
+        ` DROP INDEX IF EXISTS unique_pending_request_per_user_project_env;`,
+        callback,
+    );
+};