feat(auth): 限制同时在线 IP 数 (默认 30, 第 31 拒绝)

背景: 防 token 泄漏被滥用 + 限共享账号人数。

- 新增 app/services/active_ip.py:
    Redis ZSET 'active_ips' 存 IP,score=last_seen_unix
    登录/refresh 时 check_or_register_login_ip():
        IP 已在 set → 刷新 score,放行(老用户重连)
        IP 不在 set + ZCARD < 30 → 加入,放行
        IP 不在 set + ZCARD >= 30 → raise 429
    每个已认证请求 _resolve_user() 调 touch_ip_dependency()
        滑动 TTL,30 天没活动自动从 set 剔除
- get_client_ip() 取真实 IP,优先级 X-Forwarded-For > X-Real-IP > client.host
    trust_x_forwarded_for 默认 True(生产 Caddy/Nginx 后面)
- config 加 3 个开关:
    site_max_active_ips: int = 30
    site_active_ip_idle_days: int = 30
    trust_x_forwarded_for: bool = True
- admin.py 加 3 个端点:
    GET  /admin/active-ips       — 看当前活跃 IP 列表 + last_seen
    POST /admin/active-ips/kick  — 强制踢出指定 IP(body={ip})
    DELETE /admin/active-ips/{ip}— 简写踢出
- 注: refresh 也算 IP 占用(拿到 access token 就能用)
    但已存在的 IP 直接放行,不会踢自己

backend/app/api/admin.py

@@ -205,3 +205,58 @@ async def reset_quota(payload: QuotaReset) -> dict[str, Any]:
     key = f"translation:month:{now:%Y%m}"
     await r.set(key, payload.used_chars)
     return {"key": key, "value": payload.used_chars}
+
+
+# === 活跃 IP 看板(防 token 泄漏 / 排查异地登录)===
+class ActiveIpItem(BaseModel):
+    ip: str
+    last_seen_unix: int
+    last_seen_iso: str
+    idle_seconds: int
+
+
+class ActiveIpList(BaseModel):
+    items: list[ActiveIpItem]
+    limit: int
+    idle_days: int
+    count: int
+
+
+@router.get("/active-ips", response_model=ActiveIpList)
+async def list_active_ips_admin():
+    from app.config import settings
+    from app.services.active_ip import list_active_ips
+
+    items = await list_active_ips()
+    return ActiveIpList(
+        items=[ActiveIpItem(**i) for i in items],
+        limit=settings.site_max_active_ips,
+        idle_days=settings.site_active_ip_idle_days,
+        count=len(items),
+    )
+
+
+class KickIpRequest(BaseModel):
+    ip: str
+
+
+class KickIpResponse(BaseModel):
+    kicked: bool
+    ip: str
+
+
+@router.post("/active-ips/kick", response_model=KickIpResponse)
+async def kick_active_ip(payload: KickIpRequest):
+    """owner 强制剔除某个 IP。下次该 IP 登录时会再次被算作"新 IP"。"""
+    from app.services.active_ip import remove_active_ip
+
+    ok = await remove_active_ip(payload.ip)
+    return KickIpResponse(kicked=ok, ip=payload.ip)
+
+
+@router.delete("/active-ips/{ip}", response_model=KickIpResponse)
+async def delete_active_ip(ip: str):
+    from app.services.active_ip import remove_active_ip
+
+    ok = await remove_active_ip(ip)
+    return KickIpResponse(kicked=ok, ip=ip)