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/ai-usage.ts

@@ -0,0 +1,28 @@
+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 });
+}