feat(phase2): 일일 사용량 유틸(KST) + tarot_readings·ai_usage_log 마이그레이션

- kstDayStartISO: KST 자정을 UTC ISO로 변환
- getTodayUsage, recordUsage: AI 사용량 조회·기록
- DB: tarot_readings, ai_usage_log 테이블 생성
- saju service_settings 삭제 (숨김 해제)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AAtcmKKtqDUe4NyVgy1aLQ

lib/__tests__/ai-usage.test.ts

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+import { describe, it, expect } from 'vitest';
+import { kstDayStartISO, SAJU_DAILY_LIMIT, TAROT_DAILY_LIMIT } from '../ai-usage';
+
+describe('kstDayStartISO', () => {
+  it('KST 자정을 UTC로 환산한다 (KST 15:00 UTC = 당일 00:00 KST)', () => {
+    // 2026-07-02T05:00:00Z = 2026-07-02 14:00 KST → 그날 KST 자정 = 2026-07-01T15:00:00Z
+    expect(kstDayStartISO(new Date('2026-07-02T05:00:00Z'))).toBe('2026-07-01T15:00:00.000Z');
+  });
+  it('KST 자정 직후도 같은 날로 계산한다', () => {
+    // 2026-07-01T15:30:00Z = 2026-07-02 00:30 KST → KST 자정 = 2026-07-01T15:00:00Z
+    expect(kstDayStartISO(new Date('2026-07-01T15:30:00Z'))).toBe('2026-07-01T15:00:00.000Z');
+  });
+  it('제한 상수', () => {
+    expect(SAJU_DAILY_LIMIT).toBe(1);
+    expect(TAROT_DAILY_LIMIT).toBe(3);
+  });
+});

lib/ai-usage.ts

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+import type { SupabaseClient } from '@supabase/supabase-js';
+
+export const SAJU_DAILY_LIMIT = 1;
+export const TAROT_DAILY_LIMIT = 3;
+export type AiService = 'saju' | 'tarot';
+
+/** KST(UTC+9) 자정을 UTC ISO로. 오늘 사용량 집계 하한. */
+export function kstDayStartISO(now: Date): string {
+  const kstMs = now.getTime() + 9 * 60 * 60 * 1000;
+  const kst = new Date(kstMs);
+  const kstMidnightUtcMs = Date.UTC(kst.getUTCFullYear(), kst.getUTCMonth(), kst.getUTCDate()) - 9 * 60 * 60 * 1000;
+  return new Date(kstMidnightUtcMs).toISOString();
+}
+
+export async function getTodayUsage(admin: SupabaseClient, userId: string, service: AiService): Promise<number> {
+  const since = kstDayStartISO(new Date());
+  const { count } = await admin
+    .from('ai_usage_log')
+    .select('id', { count: 'exact', head: true })
+    .eq('user_id', userId)
+    .eq('service', service)
+    .gte('created_at', since);
+  return count ?? 0;
+}
+
+export async function recordUsage(admin: SupabaseClient, userId: string, service: AiService): Promise<void> {
+  await admin.from('ai_usage_log').insert({ user_id: userId, service });
+}

supabase/migrations/2026-07-02-phase2-saju-tarot.sql

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+-- Phase 2 (2026-07-02): 타로 저장·AI 사용량 로그 + 사주 숨김 해제
+-- 적용: 클라우드 Supabase + NAS self-host 양쪽
+
+CREATE TABLE IF NOT EXISTS tarot_readings (
+  id uuid PRIMARY KEY DEFAULT gen_random_uuid(),
+  user_id uuid NOT NULL REFERENCES auth.users(id) ON DELETE CASCADE,
+  spread_type text NOT NULL DEFAULT 'three_card',
+  category text,
+  question text,
+  cards jsonb NOT NULL,
+  interpretation jsonb NOT NULL,
+  summary text,
+  created_at timestamptz NOT NULL DEFAULT now()
+);
+ALTER TABLE tarot_readings ENABLE ROW LEVEL SECURITY;
+CREATE POLICY tarot_select_own ON tarot_readings FOR SELECT USING (auth.uid() = user_id);
+
+CREATE TABLE IF NOT EXISTS ai_usage_log (
+  id uuid PRIMARY KEY DEFAULT gen_random_uuid(),
+  user_id uuid NOT NULL,
+  service text NOT NULL CHECK (service IN ('saju','tarot')),
+  created_at timestamptz NOT NULL DEFAULT now()
+);
+ALTER TABLE ai_usage_log ENABLE ROW LEVEL SECURITY;
+CREATE INDEX IF NOT EXISTS idx_ai_usage_user_day ON ai_usage_log (user_id, service, created_at);
+
+DELETE FROM service_settings WHERE id = 'saju';