feat(rpm): RPM 限流模块优化

P0:
- rpm_override 嵌入 Auth Cache Snapshot，消除每请求 DB 查询 (snapshot v6→v7)
- 429 RPM 响应返回 Retry-After 头（当前分钟剩余秒数）

P1:
- ClearAll 按钮直连 DELETE API，带 loading 防重复
- 新增 GET /admin/users/:id/rpm-status 管理员 RPM 用量查询端点

优化:
- checkRPM 从级联互斥改为并行取最严，user.rpm_limit 作为全局硬上限始终生效
- Override/Group 变更后自动失效 auth cache
- fail-open 语义不变，Redis 故障不阻塞业务

frontend/src/components/admin/user/UserEditModal.vue

@@ -37,6 +37,18 @@
         <label class="input-label">{{ t('admin.users.columns.concurrency') }}</label>
         <input v-model.number="form.concurrency" type="number" class="input" />
       </div>
+      <div>
+        <label class="input-label">{{ t('admin.users.form.rpmLimit') }}</label>
+        <input
+          v-model.number="form.rpm_limit"
+          type="number"
+          min="0"
+          step="1"
+          class="input"
+          :placeholder="t('admin.users.form.rpmLimitPlaceholder')"
+        />
+        <p class="input-hint">{{ t('admin.users.form.rpmLimitHint') }}</p>
+      </div>
       <UserAttributeForm v-model="form.customAttributes" :user-id="user?.id" />
     </form>
     <template #footer>
@@ -66,11 +78,11 @@ const emit = defineEmits(['close', 'success'])
 const { t } = useI18n(); const appStore = useAppStore(); const { copyToClipboard } = useClipboard()
 
 const submitting = ref(false); const passwordCopied = ref(false)
-const form = reactive({ email: '', password: '', username: '', notes: '', concurrency: 1, customAttributes: {} as UserAttributeValuesMap })
+const form = reactive({ email: '', password: '', username: '', notes: '', concurrency: 1, rpm_limit: 0, customAttributes: {} as UserAttributeValuesMap })
 
 watch(() => props.user, (u) => {
   if (u) {
-    Object.assign(form, { email: u.email, password: '', username: u.username || '', notes: u.notes || '', concurrency: u.concurrency, customAttributes: {} })
+    Object.assign(form, { email: u.email, password: '', username: u.username || '', notes: u.notes || '', concurrency: u.concurrency, rpm_limit: u.rpm_limit ?? 0, customAttributes: {} })
     passwordCopied.value = false
   }
 }, { immediate: true })
@@ -97,7 +109,7 @@ const handleUpdateUser = async () => {
   }
   submitting.value = true
   try {
-    const data: any = { email: form.email, username: form.username, notes: form.notes, concurrency: form.concurrency }
+    const data: any = { email: form.email, username: form.username, notes: form.notes, concurrency: form.concurrency, rpm_limit: form.rpm_limit }
     if (form.password.trim()) data.password = form.password.trim()
     await adminAPI.users.update(props.user.id, data)
     if (Object.keys(form.customAttributes).length > 0) await adminAPI.userAttributes.updateUserAttributeValues(props.user.id, form.customAttributes)