fix(sync): 修复自托管下无法读取 Upstash 配置

app/api/user/{sync,config} 用 @upstash/redis/cloudflare 的
Redis.fromEnv() 创建客户端。该实现只读传入的 env 参数或 Cloudflare
全局绑定，从不回退到 process.env，因此在 Docker / Node 自托管下
url 与 token 恒为 undefined，两个接口必定返回 500，跨设备同步
（收藏、历史、设置）完全不可用。认证走的是 auth.ts 里的
new Redis({ url, token })，所以登录正常，只有用户数据读写失败。

把 auth.ts 中已有的双源回退抽成 lib/server/runtime-env.ts，Redis
客户端抽成 lib/server/redis.ts，两个路由改为惰性获取共享客户端。
惰性创建同时修正了另一个隐患：Cloudflare 的 per-request 绑定在
模块求值阶段不可达，而原先客户端是在模块作用域创建的。

未配置 Upstash 时返回 503 并给出明确说明，而不是当成请求失败。

Fixes #226

app/api/user/config/route.ts

@@ -5,20 +5,25 @@
  * so they persist across browsers, devices, and PWA installs.
  */
 
-import { Redis } from '@upstash/redis/cloudflare';
 import { NextRequest, NextResponse } from 'next/server';
 import { authenticationRequiredResponse } from '@/lib/server/api-responses';
 import { getServerSession } from '@/lib/server/auth';
+import { getRedisClient } from '@/lib/server/redis';
 
 export const runtime = 'edge';
 
-const redis = Redis.fromEnv();
-
 function redisKey(profileId: string): string {
   const safe = profileId.replace(/[^a-zA-Z0-9_-]/g, '');
   return `user:config:${safe}`;
 }
 
+function syncUnavailableResponse() {
+  return NextResponse.json(
+    { error: 'Server-side sync is not configured on this deployment' },
+    { status: 503 }
+  );
+}
+
 export async function GET(request: NextRequest) {
   const session = await getServerSession(request);
   const profileId = session?.profileId;
@@ -27,6 +32,11 @@ export async function GET(request: NextRequest) {
     return authenticationRequiredResponse();
   }
 
+  const redis = getRedisClient();
+  if (!redis) {
+    return syncUnavailableResponse();
+  }
+
   try {
     const data = await redis.get(redisKey(profileId));
     return NextResponse.json({ success: true, data: data || null });
@@ -47,6 +57,11 @@ export async function POST(request: NextRequest) {
     return authenticationRequiredResponse();
   }
 
+  const redis = getRedisClient();
+  if (!redis) {
+    return syncUnavailableResponse();
+  }
+
   try {
     const body = await request.json();
     const key = redisKey(profileId);

app/api/user/sync/route.ts

@@ -1,21 +1,31 @@
-import { Redis } from '@upstash/redis/cloudflare';
 import { NextRequest, NextResponse } from 'next/server';
 import { authenticationRequiredResponse } from '@/lib/server/api-responses';
 import { getServerSession } from '@/lib/server/auth';
+import { getRedisClient } from '@/lib/server/redis';
 
 // 确保这行代码在整个文件中只出现一次
 export const runtime = 'edge';
 
-const redis = Redis.fromEnv();
+function syncUnavailableResponse() {
+  return NextResponse.json(
+    { error: 'Server-side sync is not configured on this deployment' },
+    { status: 503 }
+  );
+}
 
 export async function GET(request: NextRequest) {
   const session = await getServerSession(request);
   const profileId = session?.profileId;
-  
+
   if (!profileId) {
     return authenticationRequiredResponse();
   }
 
+  const redis = getRedisClient();
+  if (!redis) {
+    return syncUnavailableResponse();
+  }
+
   try {
     const data = await redis.get(`user:sync:${profileId}`);
     return NextResponse.json({ 
@@ -31,11 +41,16 @@ export async function GET(request: NextRequest) {
 export async function POST(request: NextRequest) {
   const session = await getServerSession(request);
   const profileId = session?.profileId;
-  
+
   if (!profileId) {
     return authenticationRequiredResponse();
   }
 
+  const redis = getRedisClient();
+  if (!redis) {
+    return syncUnavailableResponse();
+  }
+
   try {
     const body = await request.json();
     const { history, favorites } = body;