[Fix] User deletion & migration

migrations/006_pending_iota_user_erasure.sql

@@ -0,0 +1,8 @@
+CREATE TABLE pending_iota_user_erasure (
+    user_id BIGINT NOT NULL,
+    iota_id BIGINT NOT NULL,
+    created_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    PRIMARY KEY (user_id, iota_id),
+    CONSTRAINT fk_pending_iota_user_erasure_iota
+        FOREIGN KEY (iota_id) REFERENCES iotas(id) ON DELETE CASCADE
+);