feat(api): album creation with translations and owner membership

Task 15: POST/GET /v1/albums, GET /v1/albums/{id}.
- create: single transaction writes albums + album_translations +
  album_members(owner) + audit_logs(album.create); publicToken 24-char
  [0-9a-z] with one P2002 retry; duplicate-locale input rejected as 400.
- dto: class-validator per contract AlbumCreateRequest, incl. cross-field
  rules (defaultLocale must be covered by translations; endAt>startAt when
  present; timezone string-only per P1a scope).
- get: PermissionsService.canViewAlbum gate -> 404 ALBUM_NOT_FOUND for
  non-member/nonexistent alike (anti-enumeration); admin bypass; joins
  categories (empty until Task 17).
- list: admin sees all, members see joined albums; stable createdAt DESC,
  id DESC keyset pagination via new shared/cursor.ts (encode/decode +
  typed where-builder, designed for Task 23 photo list reuse).
- e2e: services/api/test/albums.e2e-spec.ts (12 cases, seeded sessions).

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

services/api/src/shared/cursor.ts

@@ -0,0 +1,79 @@
+// services/api/src/shared/cursor.ts —— 键集（keyset）游标分页共用工具（Task 15）。
+// 背景：契约列表端点统一 (sortCol, id) 双列稳定排序 + CursorPageMeta（nextCursor/
+// hasMore 成对不变式）。offset 分页在并发写入下会重漏行，这里按「排序列相等时
+// 以 id 决胜」的键集谓词推进：
+//   WHERE sort < :cursorSort OR (sort = :cursorSort AND id < :cursorId)
+//   配套 ORDER BY sort DESC, id DESC（升序场景 P1a 暂无，出现时再对称扩展）。
+// 游标载体 = JSON [sortISO, id] 的 base64url（URL 查询参数安全；契约 cursor
+// maxLength 512）。Task 23 照片列表（publishedAt,id）复用本模块，勿内联重写。
+
+/** 契约 CursorQuery.maxLength。 */
+export const MAX_CURSOR_LENGTH = 512;
+
+/** 已解码的键集游标：排序列取值（ISO 8601 字符串）+ 同值决胜 id。 */
+export interface KeysetCursor {
+  readonly sort: string;
+  readonly id: string;
+}
+
+/** 编码为可回传客户端的 opaque 游标字符串。 */
+export function encodeKeysetCursor(cursor: KeysetCursor): string {
+  return Buffer.from(JSON.stringify([cursor.sort, cursor.id]), 'utf8').toString(
+    'base64url',
+  );
+}
+
+/**
+ * 解码游标；任何不合规形状（非 base64url JSON、非 [string,string] 二元组、
+ * sort 非可解析时间、超长）一律返回 null——调用方据此决定 400（显式给了
+ * cursor 却解析失败是客户端错误），本模块不抛异常、保持纯函数。
+ */
+export function decodeKeysetCursor(raw: string): KeysetCursor | null {
+  if (raw === '' || raw.length > MAX_CURSOR_LENGTH) {
+    return null;
+  }
+  let parsed: unknown;
+  try {
+    parsed = JSON.parse(Buffer.from(raw, 'base64url').toString('utf8'));
+  } catch {
+    return null;
+  }
+  if (!Array.isArray(parsed) || parsed.length !== 2) {
+    return null;
+  }
+  const [sort, id] = parsed as [unknown, unknown];
+  if (typeof sort !== 'string' || typeof id !== 'string' || id === '') {
+    return null;
+  }
+  if (Number.isNaN(Date.parse(sort))) {
+    return null;
+  }
+  return { sort, id };
+}
+
+/**
+ * 降序键集谓词（Prisma where 片段），与 ORDER BY sortKey DESC, idKey DESC 配套：
+ * 排序列严格小于游标值，或相等且 id 小于游标 id（同一排序列值内的决胜）。
+ * 泛型键名约束到具体模型的列，展开进 where 时由 Prisma 输入类型检查兜底。
+ */
+export function keysetWhereDesc<TSortKey extends PropertyKey, TIdKey extends PropertyKey>(
+  sortKey: TSortKey,
+  idKey: TIdKey,
+  cursor: KeysetCursor,
+): {
+  OR: [
+    { [K in TSortKey]: { lt: Date } },
+    { [K in TSortKey]: Date } & { [K in TIdKey]: { lt: string } },
+  ];
+} {
+  const sortValue = new Date(cursor.sort);
+  return {
+    OR: [
+      { [sortKey]: { lt: sortValue } } as { [K in TSortKey]: { lt: Date } },
+      {
+        [sortKey]: sortValue,
+        [idKey]: { lt: cursor.id },
+      } as { [K in TSortKey]: Date } & { [K in TIdKey]: { lt: string } },
+    ],
+  };
+}