feat: add SQLite-backed products store

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013gwc3aCiyQxmCWQke8RvEz

src/db.js

@@ -0,0 +1,58 @@
+const Database = require('better-sqlite3');
+const fs = require('node:fs');
+const path = require('node:path');
+
+const SCHEMA = `
+CREATE TABLE IF NOT EXISTS products (
+  id INTEGER PRIMARY KEY AUTOINCREMENT,
+  name TEXT NOT NULL,
+  url TEXT NOT NULL,
+  last_price REAL,
+  last_checked_at TEXT,
+  last_check_status TEXT,
+  created_at TEXT NOT NULL
+)`;
+
+function openDb(dbPath) {
+  if (dbPath !== ':memory:') {
+    fs.mkdirSync(path.dirname(dbPath), { recursive: true });
+  }
+  const db = new Database(dbPath);
+  db.exec(SCHEMA);
+  return db;
+}
+
+function listProducts(db) {
+  return db.prepare('SELECT * FROM products ORDER BY created_at ASC').all();
+}
+
+function getProduct(db, id) {
+  return db.prepare('SELECT * FROM products WHERE id = ?').get(id);
+}
+
+function addProduct(db, { name, url }) {
+  const createdAt = new Date().toISOString();
+  const info = db.prepare(
+    'INSERT INTO products (name, url, created_at) VALUES (?, ?, ?)'
+  ).run(name, url, createdAt);
+  return getProduct(db, info.lastInsertRowid);
+}
+
+function deleteProduct(db, id) {
+  db.prepare('DELETE FROM products WHERE id = ?').run(id);
+}
+
+function recordCheckSuccess(db, id, { price, checkedAt }) {
+  db.prepare(
+    'UPDATE products SET last_price = ?, last_checked_at = ?, last_check_status = ? WHERE id = ?'
+  ).run(price, checkedAt, 'ok', id);
+}
+
+function recordCheckFailure(db, id, status) {
+  db.prepare('UPDATE products SET last_check_status = ? WHERE id = ?').run(status, id);
+}
+
+module.exports = {
+  openDb, listProducts, getProduct, addProduct, deleteProduct,
+  recordCheckSuccess, recordCheckFailure,
+};