feat: 添加提供商格式转换优先级保持配置

- 新增 Provider.keep_priority_on_conversion 字段，控制格式转换时是否保持优先级
- 新增全局配置 KEEP_PRIORITY_ON_CONVERSION，可全局启用优先级保持
- 调度器根据配置决定是否降级需要格式转换的候选
- 前端 Provider 表单添加"保持优先级"开关
- 调整 HTTP 读写超时默认值为 3600 秒

frontend/src/api/endpoints/types.ts

@@ -309,6 +309,7 @@ export interface ProviderWithEndpointsSummary {
   description?: string
   website?: string
   provider_priority: number
+  keep_priority_on_conversion: boolean  // 格式转换时是否保持优先级
   billing_type?: 'monthly_quota' | 'pay_as_you_go' | 'free_tier'
   monthly_quota_usd?: number
   monthly_used_usd?: number

frontend/src/features/providers/components/ProviderFormDialog.vue

@@ -171,6 +171,25 @@
         </div>
       </div>
 
+      <!-- 格式转换配置 -->
+      <div class="space-y-3">
+        <h3 class="text-sm font-medium border-b pb-2">
+          格式转换
+        </h3>
+        <div class="flex items-center justify-between p-3 border rounded-lg bg-muted/50">
+          <div class="space-y-0.5">
+            <span class="text-sm font-medium">保持优先级</span>
+            <p class="text-xs text-muted-foreground">
+              跨格式请求时保持原优先级排名，不降级到格式匹配的提供商之后
+            </p>
+          </div>
+          <Switch
+            :model-value="form.keep_priority_on_conversion"
+            @update:model-value="(v: boolean) => form.keep_priority_on_conversion = v"
+          />
+        </div>
+      </div>
+
       <!-- 代理配置 -->
       <div class="space-y-3">
         <div class="flex items-center justify-between">
@@ -295,6 +314,7 @@ const form = ref({
   quota_last_reset_at: '',  // 周期开始时间
   quota_expires_at: '',
   provider_priority: 999,
+  keep_priority_on_conversion: false,  // 格式转换时是否保持优先级
   // 状态配置
   is_active: true,
   rate_limit: undefined as number | undefined,
@@ -323,6 +343,7 @@ function resetForm() {
     quota_last_reset_at: '',
     quota_expires_at: '',
     provider_priority: 999,
+    keep_priority_on_conversion: false,
     is_active: true,
     rate_limit: undefined,
     concurrent_limit: undefined,
@@ -356,6 +377,7 @@ function loadProviderData() {
     quota_expires_at: props.provider.quota_expires_at ?
       new Date(props.provider.quota_expires_at).toISOString().slice(0, 16) : '',
     provider_priority: props.provider.provider_priority || 999,
+    keep_priority_on_conversion: props.provider.keep_priority_on_conversion ?? false,
     is_active: props.provider.is_active,
     rate_limit: undefined,
     concurrent_limit: undefined,
@@ -416,6 +438,7 @@ const handleSubmit = async () => {
       quota_last_reset_at: form.value.quota_last_reset_at || undefined,
       quota_expires_at: form.value.quota_expires_at || undefined,
       provider_priority: form.value.provider_priority,
+      keep_priority_on_conversion: form.value.keep_priority_on_conversion,
       is_active: form.value.is_active,
       // 请求配置
       max_retries: form.value.max_retries ?? undefined,