feat(rate-limit): 实现分层 RPM 限速，支持系统默认/用户/独立Key三级配置

- 新增用户级 rate_limit 字段，支持系统默认/用户自定义/不限制三种模式
- 独立 Key 的 rate_limit 语义调整：null=跟随系统默认，0=不限制，>0=自定义
- 实现 UserRpmLimiter 基于 Redis sliding window 的 RPM 限速引擎
- Pipeline 请求流程集成用户级 RPM 检查
- 管理后台和用户面板新增 RPM 限速配置与实时状态查看
- 系统设置新增全局默认 RPM 配置项
- 迁移脚本回填现有 API Key 的 rate_limit 默认值
- 新增用户/Key RPM 状态监控 API 和前端展示

Closes #231

Co-authored-by: LewisPen <LewisPen@nyadoo.com>

frontend/src/features/api-keys/components/StandaloneKeyFormDialog.vue

@@ -96,23 +96,6 @@
               {{ form.expires_at ? '到期后' + (form.auto_delete_on_expiry ? '自动删除' : '仅禁用') + '（当天 23:59 失效）' : '留空表示永不过期' }}
             </p>
           </div>
-
-          <div class="space-y-2">
-            <Label
-              for="form-rate-limit"
-              class="text-sm font-medium"
-            >速率限制 (请求/分钟)</Label>
-            <Input
-              id="form-rate-limit"
-              :model-value="form.rate_limit ?? ''"
-              type="number"
-              min="1"
-              max="10000"
-              placeholder="留空不限制"
-              class="h-10"
-              @update:model-value="(v) => form.rate_limit = parseNumberInput(v, { min: 1, max: 10000 })"
-            />
-          </div>
         </div>
 
         <!-- 右侧：访问限制 -->
@@ -190,6 +173,36 @@
             </div>
           </div>
 
+          <div class="space-y-2">
+            <Label
+              for="form-rate-limit"
+              class="text-sm font-medium"
+            >速率限制 (请求/分钟)</Label>
+            <div class="flex items-center gap-3">
+              <div class="flex-1 min-w-0">
+                <Input
+                  v-if="!form.rate_limit_inherited"
+                  id="form-rate-limit"
+                  :model-value="form.rate_limit ?? ''"
+                  type="number"
+                  min="0"
+                  max="10000"
+                  placeholder="0 = 不限速"
+                  class="h-10"
+                  @update:model-value="(v) => form.rate_limit = parseNumberInput(v, { min: 0, max: 10000 })"
+                />
+                <span
+                  v-else
+                  class="flex h-10 w-full items-center rounded-lg border bg-background px-3 text-sm text-muted-foreground opacity-60"
+                >跟随系统</span>
+              </div>
+              <Switch
+                v-model="form.rate_limit_inherited"
+                class="shrink-0"
+              />
+            </div>
+          </div>
+
           <!-- 额度 -->
           <div class="space-y-2">
             <Label class="text-sm font-medium">额度</Label>
@@ -267,7 +280,7 @@ export interface StandaloneKeyFormData {
   initial_balance_usd?: number
   unlimited_balance?: boolean
   expires_at?: string  // ISO 日期字符串，如 "2025-12-31"，undefined = 永不过期
-  rate_limit?: number
+  rate_limit?: number | null
   auto_delete_on_expiry: boolean
   allowed_providers?: string[] | null
   allowed_api_formats?: string[] | null
@@ -280,6 +293,7 @@ interface StandaloneKeyFormState {
   initial_balance_usd?: number
   unlimited_balance?: boolean
   expires_at?: string
+  rate_limit_inherited: boolean
   rate_limit?: number
   auto_delete_on_expiry: boolean
   provider_unrestricted: boolean
@@ -333,6 +347,7 @@ const form = ref<StandaloneKeyFormState>({
   initial_balance_usd: 10,
   unlimited_balance: false,
   expires_at: undefined,
+  rate_limit_inherited: true,
   rate_limit: undefined,
   auto_delete_on_expiry: false,
   provider_unrestricted: true,
@@ -356,6 +371,7 @@ function resetForm() {
     initial_balance_usd: 10,
     unlimited_balance: false,
     expires_at: undefined,
+    rate_limit_inherited: true,
     rate_limit: undefined,
     auto_delete_on_expiry: false,
     provider_unrestricted: true,
@@ -375,7 +391,8 @@ function loadKeyData() {
     initial_balance_usd: props.apiKey.initial_balance_usd,
     unlimited_balance: props.apiKey.initial_balance_usd == null,
     expires_at: props.apiKey.expires_at,
-    rate_limit: props.apiKey.rate_limit,
+    rate_limit_inherited: props.apiKey.rate_limit == null,
+    rate_limit: props.apiKey.rate_limit ?? undefined,
     auto_delete_on_expiry: props.apiKey.auto_delete_on_expiry,
     provider_unrestricted: props.apiKey.allowed_providers == null,
     api_format_unrestricted: props.apiKey.allowed_api_formats == null,
@@ -425,7 +442,7 @@ function handleSubmit() {
     initial_balance_usd: form.value.initial_balance_usd,
     unlimited_balance: form.value.unlimited_balance,
     expires_at: form.value.expires_at,
-    rate_limit: form.value.rate_limit,
+    rate_limit: form.value.rate_limit_inherited ? null : (form.value.rate_limit ?? 0),
     auto_delete_on_expiry: form.value.auto_delete_on_expiry,
     allowed_providers: form.value.provider_unrestricted ? null : [...form.value.allowed_providers],
     allowed_api_formats: form.value.api_format_unrestricted ? null : [...form.value.allowed_api_formats],