feat: database layer with better-sqlite3, full schema and query functions

Implements Task 4 (TDD): wrote failing tests first, then the full SQLite
database layer covering profiles, magic tokens, sessions, user progress,
mnemonics, and letter stats. All 14 tests pass.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server/tests/db.test.js

@@ -0,0 +1,142 @@
+'use strict'
+const { test, describe, before, after } = require('node:test')
+const assert = require('node:assert/strict')
+const { initDb } = require('../db.js')
+
+let db
+
+before(() => {
+  db = initDb(':memory:')
+})
+
+after(() => {
+  db._db.close()
+})
+
+describe('profiles', () => {
+  test('createProfile creates a profile', () => {
+    const p = db.createProfile('test@example.com', 'Tester')
+    assert.equal(p.email, 'test@example.com')
+    assert.equal(p.display_name, 'Tester')
+    assert.ok(p.id)
+  })
+
+  test('findProfileByEmail returns the profile', () => {
+    const p = db.findProfileByEmail('test@example.com')
+    assert.equal(p.email, 'test@example.com')
+  })
+
+  test('findProfileByEmail returns null for unknown email', () => {
+    assert.equal(db.findProfileByEmail('nobody@example.com'), null)
+  })
+})
+
+describe('magic tokens', () => {
+  let profileId
+
+  before(() => {
+    profileId = db.createProfile('magic@example.com', 'Magic').id
+  })
+
+  test('createMagicToken and useMagicToken returns profileId', () => {
+    const expires = Date.now() + 86400000
+    db.createMagicToken('tok123', profileId, expires)
+    const result = db.useMagicToken('tok123')
+    assert.equal(result, profileId)
+  })
+
+  test('useMagicToken returns null for used token', () => {
+    assert.equal(db.useMagicToken('tok123'), null)
+  })
+
+  test('useMagicToken returns null for expired token', () => {
+    db.createMagicToken('expiredtok', profileId, Date.now() - 1000)
+    assert.equal(db.useMagicToken('expiredtok'), null)
+  })
+})
+
+describe('sessions', () => {
+  let profileId
+
+  before(() => {
+    profileId = db.createProfile('session@example.com', 'Session').id
+  })
+
+  test('createSession and getSession returns profile_id', () => {
+    db.createSession('sess1', profileId)
+    const s = db.getSession('sess1')
+    assert.equal(s.profile_id, profileId)
+  })
+
+  test('getSession returns null for unknown token', () => {
+    assert.equal(db.getSession('unknown'), null)
+  })
+
+  test('deleteSession removes session', () => {
+    db.deleteSession('sess1')
+    assert.equal(db.getSession('sess1'), null)
+  })
+})
+
+describe('progress', () => {
+  let profileId
+
+  before(() => {
+    profileId = db.createProfile('progress@example.com', 'Progress').id
+  })
+
+  test('getOrCreateProgress returns default progress', () => {
+    const p = db.getOrCreateProgress(profileId)
+    assert.equal(p.level, 1)
+    assert.equal(p.score, 0)
+    assert.equal(p.streak, 0)
+  })
+
+  test('recordAnswer increments score and streak on correct', () => {
+    db.recordAnswer(profileId, 'A', true)
+    const p = db.getOrCreateProgress(profileId)
+    assert.ok(p.score > 0)
+    assert.equal(p.streak, 1)
+    assert.equal(p.total_correct, 1)
+    assert.equal(p.total_attempts, 1)
+  })
+
+  test('recordAnswer resets streak on incorrect', () => {
+    db.recordAnswer(profileId, 'A', false)
+    const p = db.getOrCreateProgress(profileId)
+    assert.equal(p.streak, 0)
+    assert.equal(p.total_attempts, 2)
+  })
+})
+
+describe('mnemonics', () => {
+  let profileId
+
+  before(() => {
+    profileId = db.createProfile('mnemonic@example.com', 'Mnemonic').id
+  })
+
+  test('saveMnemonic and getMnemonics returns map', () => {
+    db.saveMnemonic(profileId, 'A', 'go WALKING')
+    db.saveMnemonic(profileId, 'E', 'go')
+    const m = db.getMnemonics(profileId)
+    assert.equal(m.A, 'go WALKING')
+    assert.equal(m.E, 'go')
+  })
+})
+
+describe('letter stats', () => {
+  let profileId
+
+  before(() => {
+    profileId = db.createProfile('stats@example.com', 'Stats').id
+  })
+
+  test('recordAnswer tracks letter stats', () => {
+    db.recordAnswer(profileId, 'S', true)
+    db.recordAnswer(profileId, 'S', false)
+    const stats = db.getLetterStats(profileId)
+    assert.equal(stats.S.correct, 1)
+    assert.equal(stats.S.attempts, 2)
+  })
+})