feat: make user limit configurable from admin user management (#18)

Store the per-instance user cap in settings (default 20) and expose
GET/PUT /admin/users/limit endpoints. The user management page now lets
admins view and update the limit without touching system settings.

Co-authored-by: Cursor Agent <cursoragent@cursor.com>
Co-authored-by: truewhile <truewhile@users.noreply.github.com>

web/src/pages/AdminUsersForm.tsx

@@ -1,12 +1,16 @@
 import { FormEvent } from 'react'
-import { Plus } from 'lucide-react'
+import { Plus, Save } from 'lucide-react'
 
 type AdminUsersFormProps = {
   usersCount: number
-  userLimitLabel: string
+  maxUsers: number
+  maxUsersDraft: string
+  savingLimit: boolean
   username: string
   password: string
   userLimitReached: boolean
+  onMaxUsersDraftChange: (value: string) => void
+  onSaveMaxUsers: () => void
   onUsernameChange: (value: string) => void
   onPasswordChange: (value: string) => void
   onSubmit: (e: FormEvent) => void
@@ -14,27 +18,61 @@ type AdminUsersFormProps = {
 
 export function AdminUsersForm({
   usersCount,
-  userLimitLabel,
+  maxUsers,
+  maxUsersDraft,
+  savingLimit,
   username,
   password,
   userLimitReached,
+  onMaxUsersDraftChange,
+  onSaveMaxUsers,
   onUsernameChange,
   onPasswordChange,
   onSubmit,
 }: AdminUsersFormProps) {
+  const draftValue = Number(maxUsersDraft)
+  const limitChanged = maxUsersDraft.trim() !== '' && Number.isFinite(draftValue) && draftValue !== maxUsers
+
   return (
     <form onSubmit={onSubmit} className="glass-panel grid gap-3 md:grid-cols-[1fr_1fr_auto]">
       <div className="md:col-span-3 flex flex-wrap items-center justify-between gap-3">
         <div>
           <h2 className="font-display text-lg font-semibold text-ink-600">用户管理</h2>
           <p className="text-xs text-sand-500">
-            已创建 {usersCount}/{userLimitLabel} 个用户；新增用户默认只有媒体库浏览、播放、外部播放器与第三方客户端观看权限。
+            已创建 {usersCount}/{maxUsers} 个用户；新增用户默认只有媒体库浏览、播放、外部播放器与第三方客户端观看权限。
           </p>
         </div>
         <span className="rounded-full border border-primary-400/30 px-3 py-1 text-xs text-brand-500">
           默认管理员不可删除 · 最高权限
         </span>
       </div>
+
+      <div className="md:col-span-3 flex flex-wrap items-end gap-3 rounded-2xl border border-sand-200/80 bg-white/60 p-4">
+        <label className="min-w-[10rem] flex-1">
+          <span className="mb-1 block text-xs font-medium text-sand-500">单实例用户数量上限</span>
+          <input
+            type="number"
+            min={1}
+            max={10000}
+            className="input-base"
+            value={maxUsersDraft}
+            onChange={(e) => onMaxUsersDraftChange(e.target.value)}
+          />
+        </label>
+        <button
+          type="button"
+          className="neon-button inline-flex items-center justify-center gap-2"
+          disabled={!limitChanged || savingLimit || !Number.isFinite(draftValue) || draftValue < 1}
+          onClick={onSaveMaxUsers}
+        >
+          <Save size={16} />
+          {savingLimit ? '保存中…' : '保存上限'}
+        </button>
+        <p className="w-full text-xs text-sand-500">
+          默认 20 人。达到上限后无法继续注册或添加用户；若当前用户数已超过新上限，已有账号不受影响，但需删除部分用户后才能继续添加。
+        </p>
+      </div>
+
       <input
         required
         className="input-base"