feat: add versioned migrations and kysely query layer

backend/migrations/1700000008000_create-trip-preference-overrides.cjs

@@ -0,0 +1,30 @@
+exports.up = (pgm) => {
+  pgm.createTable('trip_preference_overrides', {
+    id: { type: 'uuid', primaryKey: true, default: pgm.func('gen_random_uuid()') },
+    trip_id: { type: 'uuid', notNull: true, references: 'trips(id)', onDelete: 'CASCADE' },
+    user_id: { type: 'uuid', references: 'users(id)' },
+    traveler_id: { type: 'uuid', references: 'travelers(id)', onDelete: 'CASCADE' },
+    overrides: { type: 'jsonb', notNull: true, default: '{}' },
+    created_at: { type: 'timestamptz', notNull: true, default: pgm.func('now()') },
+    updated_at: { type: 'timestamptz', notNull: true, default: pgm.func('now()') },
+  });
+  pgm.addConstraint(
+    'trip_preference_overrides',
+    'trip_preference_overrides_subject_check',
+    'CHECK (num_nonnulls(user_id, traveler_id) = 1)',
+  );
+  pgm.createIndex('trip_preference_overrides', ['trip_id', 'user_id'], {
+    unique: true,
+    where: 'user_id IS NOT NULL',
+    name: 'trip_pref_overrides_trip_user_uq',
+  });
+  pgm.createIndex('trip_preference_overrides', ['trip_id', 'traveler_id'], {
+    unique: true,
+    where: 'traveler_id IS NOT NULL',
+    name: 'trip_pref_overrides_trip_traveler_uq',
+  });
+};
+
+exports.down = (pgm) => {
+  pgm.dropTable('trip_preference_overrides');
+};