refactor: 清理 13 处生产死代码

为什么：用 AST 级引用分析（而非文本 grep——同名符号会互相掩盖）确认这些
符号全项目零引用。文本 grep 查不出的原因是它们都有「活着的同名兄弟」：

services/downloads.py（共 6 个，93 行）
  _parse_cookie_str / _global_music_u / _vip_name / _detect_vip /
  _admin_client / _user_client
  —— 活的是 core.media.parse_cookie_str、web/routers/music.py、account.py 的版本
  随之失效的 import：USER_COOKIE_FILE / GLOBAL_MUSIC_U_CACHE / VIP_PROBE_LOCK /
  account_info / detect_vip_by_probe / QRLoginClient

services/api_service.py
  APIResponse 类（29 行）—— web/response.py 才是活的响应封装

services/_shared.py
  _rate_ok —— 逐字复制 web.ratelimit.rate_ok

core/utils.py
  safe_int / safe_console_print —— 活的是 web/request_utils.safe_int

core/repo.py
  all_settings / get_user_by_name —— auth_system.get_user_by_name 才是活的

core/db.py / web/auth_state.py / web/request_utils.py
  execute_many / _safe_register_credits + secrets_rand / dedup_ids

结果：生产代码死代码候选 13 → 0（余下 118 个全是 pytest 测试函数，属正常）。

core/repo.py

@@ -28,11 +28,6 @@ async def set_setting(key: str, value: str) -> None:
     )
 
 
-async def all_settings() -> dict[str, str]:
-    rows = await db.fetch_all('SELECT key, value FROM settings')
-    return {r['key']: r['value'] for r in rows}
-
-
 # ---------- announcements ----------
 
 async def list_announcements(limit: int = 20) -> list[dict[str, Any]]:
@@ -49,11 +44,6 @@ async def count_users(role: str | None = None) -> int:
     return int(await db.fetch_val('SELECT COUNT(*) FROM users', (), 0))
 
 
-async def get_user_by_name(username: str) -> dict[str, Any] | None:
-    row = await db.fetch_one('SELECT * FROM users WHERE username = ?', (username,))
-    return dict(row) if row is not None else None
-
-
 async def get_user_by_id(uid: int) -> dict[str, Any] | None:
     row = await db.fetch_one('SELECT * FROM users WHERE id = ?', (uid,))
     return dict(row) if row is not None else None