feat: add versioned migrations and kysely query layer

backend/migrations/1700000007000_create-travelers.cjs

@@ -0,0 +1,22 @@
+exports.up = (pgm) => {
+  pgm.createTable('travelers', {
+    id: { type: 'uuid', primaryKey: true, default: pgm.func('gen_random_uuid()') },
+    trip_id: { type: 'uuid', notNull: true, references: 'trips(id)', onDelete: 'CASCADE' },
+    linked_user_id: { type: 'uuid', references: 'users(id)' },
+    display_name: { type: 'text', notNull: true },
+    traveler_type: { type: 'text', notNull: true },
+    created_by_user_id: { type: 'uuid', notNull: true, references: 'users(id)' },
+    created_at: { type: 'timestamptz', notNull: true, default: pgm.func('now()') },
+    updated_at: { type: 'timestamptz', notNull: true, default: pgm.func('now()') },
+  });
+  pgm.addConstraint(
+    'travelers',
+    'travelers_traveler_type_check',
+    "CHECK (traveler_type IN ('ADULT','CHILD','INFANT'))",
+  );
+  pgm.createIndex('travelers', 'trip_id');
+};
+
+exports.down = (pgm) => {
+  pgm.dropTable('travelers');
+};