feat(auth): 登录页(用户名/密码换 API token)+ 前端全链路登录态

后端:
- POST /api/auth/login:admin 账密(Settings 默认 admin/admin123,
  ADMIN_USER/ADMIN_PASSWORD 可覆盖)换取共享 api_token,免 token 中间件
- 防爆破:同 IP 15min 窗口内 5 次失败锁 15min(429),成功登录清零
- 错误信息统一「用户名或密码错误」(防用户名枚举)
- 测试 7 个新增(TDD),全量 500 绿

前端:
- LoginView + 路由守卫(未登录跳 /login,回跳 redirect)
- axios 请求拦截器自动带 Bearer;401 清登录态回登录页
- 媒体直链统一 mediaUrl() 带 ?token=(img/audio 标签发不了 header),
  替换 4 处裸拼;downloadBlob 手动带头
- isLoggedIn 按 key 存在判定 — 后端鉴权关闭时空 token 不锁死登录页
- MainLayout 侧栏底部退出登录

compose:两份编排透传 ADMIN_USER/ADMIN_PASSWORD(默认 admin/admin123)

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

yunke-server/app/api/auth.py

@@ -19,15 +19,47 @@ reach it without knowing the token — that exemption is encoded here in
 from __future__ import annotations
 
 import secrets
+import time
 
 from fastapi import HTTPException, Request, status
 
 from app.config import get_settings
 
 # Request paths that bypass auth even when a token is configured. Keep this
-# minimal so the protected surface is easy to reason about — currently just the
-# liveness probe.
-_PUBLIC_PATHS = frozenset({"/api/health"})
+# minimal so the protected surface is easy to reason about — the liveness
+# probe and the login endpoint itself (the caller has no token yet).
+_PUBLIC_PATHS = frozenset({"/api/health", "/api/auth/login"})
+
+# ---- Brute-force guard for POST /api/auth/login ------------------------- #
+# In-memory per-IP failure tracker: MAX failures within WINDOW seconds locks
+# the IP out for LOCKOUT seconds. Process-local by design (single-backend
+# deployment; a restart simply clears the slate).
+_LOGIN_MAX_FAILURES = 5
+_LOGIN_FAILURE_WINDOW_S = 15 * 60
+_LOGIN_LOCKOUT_S = 15 * 60
+# ip -> list of monotonic timestamps of recent failed logins
+_login_failures: dict[str, list[float]] = {}
+
+
+def _client_ip(request: Request) -> str:
+    """Best-effort client IP for the lockout key (test transport has none)."""
+    return request.client.host if request.client else "unknown"
+
+
+def _login_locked_out(ip: str) -> bool:
+    """True when ``ip`` burned through MAX failures inside the window."""
+    now = time.monotonic()
+    stamps = [t for t in _login_failures.get(ip, []) if now - t < _LOGIN_FAILURE_WINDOW_S]
+    _login_failures[ip] = stamps
+    return len(stamps) >= _LOGIN_MAX_FAILURES
+
+
+def _record_login_failure(ip: str) -> None:
+    _login_failures.setdefault(ip, []).append(time.monotonic())
+
+
+def _reset_login_failures(ip: str) -> None:
+    _login_failures.pop(ip, None)
 
 
 async def require_auth(request: Request) -> None: