feat(db): drizzle schema, migrations, and seed

packages/backend/src/db/client.ts

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+import Database from 'better-sqlite3';
+import { drizzle, type BetterSQLite3Database } from 'drizzle-orm/better-sqlite3';
+import { mkdirSync } from 'node:fs';
+import { dirname, resolve } from 'node:path';
+import * as schema from './schema.js';
+
+export type Db = BetterSQLite3Database<typeof schema>;
+
+export function createDb(dbPath?: string): { db: Db; sqlite: Database.Database } {
+  const path = dbPath ?? process.env.DB_PATH ?? resolve(process.cwd(), '../../data/flashcard.db');
+  if (path !== ':memory:') {
+    mkdirSync(dirname(path), { recursive: true });
+  }
+  const sqlite = new Database(path);
+  sqlite.pragma('journal_mode = WAL');
+  sqlite.pragma('foreign_keys = ON');
+  const db = drizzle(sqlite, { schema });
+  return { db, sqlite };
+}

packages/backend/src/db/migrate.ts

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+import { migrate } from 'drizzle-orm/better-sqlite3/migrator';
+import { resolve } from 'node:path';
+import { createDb } from './client.js';
+
+const { db, sqlite } = createDb();
+migrate(db, { migrationsFolder: resolve(import.meta.dirname, '../../drizzle') });
+sqlite.close();
+console.log('Migrations applied.');

packages/backend/src/db/schema.ts

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+import { sql } from 'drizzle-orm';
+import { integer, sqliteTable, text, index } from 'drizzle-orm/sqlite-core';
+
+export const lessons = sqliteTable('lessons', {
+  id: integer('id').primaryKey({ autoIncrement: true }),
+  parentId: integer('parent_id'),
+  name: text('name').notNull(),
+  description: text('description'),
+  position: integer('position').notNull().default(0),
+  bidirectional: integer('bidirectional', { mode: 'boolean' }).notNull().default(false),
+  createdAt: integer('created_at').notNull().default(sql`(unixepoch())`),
+  updatedAt: integer('updated_at').notNull().default(sql`(unixepoch())`),
+});
+
+export const cards = sqliteTable(
+  'cards',
+  {
+    id: integer('id').primaryKey({ autoIncrement: true }),
+    lessonId: integer('lesson_id').notNull().references(() => lessons.id, { onDelete: 'cascade' }),
+    question: text('question').notNull(),
+    answer: text('answer').notNull(),
+    hint: text('hint'),
+    position: integer('position').notNull().default(0),
+    createdAt: integer('created_at').notNull().default(sql`(unixepoch())`),
+    updatedAt: integer('updated_at').notNull().default(sql`(unixepoch())`),
+  },
+  (t) => ({ lessonIdx: index('cards_lesson_idx').on(t.lessonId) })
+);
+
+export const cardProgress = sqliteTable(
+  'card_progress',
+  {
+    cardId: integer('card_id').notNull().references(() => cards.id, { onDelete: 'cascade' }),
+    direction: text('direction', { enum: ['forward', 'backward'] }).notNull(),
+    box: integer('box').notNull().default(1),
+    correctCount: integer('correct_count').notNull().default(0),
+    incorrectCount: integer('incorrect_count').notNull().default(0),
+    lastShownAt: integer('last_shown_at'),
+    nextDueAt: integer('next_due_at').notNull().default(0),
+  },
+  (t) => ({
+    pk: index('card_progress_pk').on(t.cardId, t.direction),
+    dueIdx: index('card_progress_due_idx').on(t.nextDueAt),
+  })
+);
+
+export const sessions = sqliteTable(
+  'sessions',
+  {
+    id: integer('id').primaryKey({ autoIncrement: true }),
+    lessonId: integer('lesson_id').notNull().references(() => lessons.id, { onDelete: 'cascade' }),
+    startedAt: integer('started_at').notNull().default(sql`(unixepoch())`),
+    endedAt: integer('ended_at'),
+    durationSeconds: integer('duration_seconds'),
+    cardsShown: integer('cards_shown').notNull().default(0),
+    cardsCorrect: integer('cards_correct').notNull().default(0),
+    cardsIncorrect: integer('cards_incorrect').notNull().default(0),
+    status: text('status', { enum: ['active', 'completed', 'abandoned'] }).notNull().default('active'),
+    queueSnapshot: text('queue_snapshot'),
+  },
+  (t) => ({ statusIdx: index('sessions_status_idx').on(t.status) })
+);
+
+export const attempts = sqliteTable(
+  'attempts',
+  {
+    id: integer('id').primaryKey({ autoIncrement: true }),
+    sessionId: integer('session_id').notNull().references(() => sessions.id, { onDelete: 'cascade' }),
+    cardId: integer('card_id').notNull().references(() => cards.id, { onDelete: 'cascade' }),
+    direction: text('direction', { enum: ['forward', 'backward'] }).notNull(),
+    shownAt: integer('shown_at').notNull().default(sql`(unixepoch())`),
+    result: text('result', { enum: ['correct', 'incorrect'] }).notNull(),
+    timeToAnswerMs: integer('time_to_answer_ms'),
+  },
+  (t) => ({
+    sessionIdx: index('attempts_session_idx').on(t.sessionId),
+    cardIdx: index('attempts_card_idx').on(t.cardId),
+  })
+);
+
+export type LessonRow = typeof lessons.$inferSelect;
+export type CardRow = typeof cards.$inferSelect;
+export type CardProgressRow = typeof cardProgress.$inferSelect;
+export type SessionRow = typeof sessions.$inferSelect;
+export type AttemptRow = typeof attempts.$inferSelect;

packages/backend/src/db/seed.ts

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+import { createDb } from './client.js';
+import { cards, lessons } from './schema.js';
+
+const { db, sqlite } = createDb();
+const [root] = db.insert(lessons).values({ name: 'Demo: Spaans', position: 0 }).returning().all();
+const [sub] = db.insert(lessons).values({ name: 'Begroetingen', parentId: root.id, position: 0 }).returning().all();
+db.insert(cards).values([
+  { lessonId: sub.id, question: 'Hallo', answer: 'Hola', position: 0 },
+  { lessonId: sub.id, question: 'Goedemorgen', answer: 'Buenos días', position: 1 },
+  { lessonId: sub.id, question: 'Tot ziens', answer: 'Adiós', position: 2 },
+]).run();
+sqlite.close();
+console.log('Seed inserted.');