feat(packs): lib helpers — pack-files supabase 쿼리 + web-backend HMAC 클라이언트

- pack-files.ts: tier hierarchy + getPackFilesForTiers + getPackFileById
- web-backend.ts: signLink (HMAC sig) + mintUploadToken (일회성 jti, 15분 만료)
  + listPackFilesViaBackend + deletePackFileViaBackend

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

lib/supabase/pack-files.ts

@@ -0,0 +1,54 @@
+import type { SupabaseClient } from '@supabase/supabase-js';
+import type { PackTier } from '../pack-assets';
+
+export interface PackFile {
+  id: string;
+  min_tier: PackTier;
+  label: string;
+  file_path: string;
+  filename: string;
+  size_bytes: number;
+  sort_order: number;
+  uploaded_at: string;
+  deleted_at: string | null;
+}
+
+const TIER_HIERARCHY: Record<PackTier, PackTier[]> = {
+  starter: ['starter'],
+  pro: ['starter', 'pro'],
+  master: ['starter', 'pro', 'master'],
+};
+
+export function tierIncludes(userTier: PackTier): PackTier[] {
+  return TIER_HIERARCHY[userTier];
+}
+
+export async function getPackFilesForTiers(
+  supabase: SupabaseClient,
+  tiers: PackTier[],
+): Promise<PackFile[]> {
+  if (tiers.length === 0) return [];
+  const { data, error } = await supabase
+    .from('pack_files')
+    .select('*')
+    .in('min_tier', tiers)
+    .is('deleted_at', null)
+    .order('min_tier')
+    .order('sort_order');
+  if (error) throw error;
+  return (data ?? []) as PackFile[];
+}
+
+export async function getPackFileById(
+  supabase: SupabaseClient,
+  id: string,
+): Promise<PackFile | null> {
+  const { data, error } = await supabase
+    .from('pack_files')
+    .select('*')
+    .eq('id', id)
+    .is('deleted_at', null)
+    .maybeSingle();
+  if (error) throw error;
+  return (data as PackFile) ?? null;
+}