[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;

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
+);