fix: remove explicit transaction from migration 012

Goose wraps each migration in a transaction automatically.
Explicit BEGIN TRANSACTION/COMMIT caused a nested transaction
error in SQLite.

internal/database/migrations/012_remove_recovery_key.sql

@@ -1,8 +1,6 @@
 -- +goose Up
 PRAGMA foreign_keys = OFF;
 
-BEGIN TRANSACTION;
-
 CREATE TABLE user_new (
   id TEXT PRIMARY KEY,
   username TEXT NOT NULL UNIQUE,
@@ -18,8 +16,6 @@ DROP TABLE user;
 
 ALTER TABLE user_new RENAME TO user;
 
-COMMIT;
-
 PRAGMA foreign_keys = ON;
 
 -- +goose Down