feat: add versioned migrations and kysely query layer

backend/migrations/1700000001000_create-users.cjs

@@ -0,0 +1,16 @@
+exports.up = (pgm) => {
+  pgm.createExtension('pgcrypto', { ifNotExists: true });
+  pgm.createTable('users', {
+    id: { type: 'uuid', primaryKey: true, default: pgm.func('gen_random_uuid()') },
+    external_subject_id: { type: 'text', notNull: true },
+    display_name: { type: 'text', notNull: true },
+    email: { type: 'text', notNull: true },
+    created_at: { type: 'timestamptz', notNull: true, default: pgm.func('now()') },
+    updated_at: { type: 'timestamptz', notNull: true, default: pgm.func('now()') },
+  });
+  pgm.addConstraint('users', 'users_external_subject_id_key', 'UNIQUE(external_subject_id)');
+};
+
+exports.down = (pgm) => {
+  pgm.dropTable('users');
+};