[Fix] User deletion & migration

migrations/005_nullable_user_iota.sql

@@ -0,0 +1,11 @@
+ALTER TABLE users
+    DROP FOREIGN KEY fk_users_iota;
+
+ALTER TABLE users
+    MODIFY iota_id BIGINT NULL;
+
+ALTER TABLE users
+    ADD CONSTRAINT fk_users_iota
+        FOREIGN KEY (iota_id)
+        REFERENCES iotas (id)
+        ON DELETE SET NULL;