feat: full-text search (messages + customers)

Backend (yunke-server):
- GET /api/search?q=&type=messages|customers|all&channel=&start=&end=&page=&page_size=
  LIKE '%q%' scan over message.text + customer friend_nickname/real_name/
  phone/wechat_phone. type=all returns both sections; each is its own Page[T].
- type=messages orders by timestamp desc; type=customers orders by
  last_chat_time desc (nulls last).
- q required: missing/empty/whitespace -> HTTP 400.
- 14 new tests in tests/test_api_search.py (TDD red -> green).

Frontend (yunkes):
- New /search route + 搜索 nav item with Search icon.
- SearchView.vue: search bar, two stacked sections (消息结果 / 客户结果),
  counts + independent pagination per section, keyword highlighting (escaped),
  click message -> /chat/:channel/:ownerId/:talkerId,
  click customer -> all-fields drawer (CustomerList pattern reuse).
- search() wrapper in src/api/index.ts with SearchQuery/SearchPayload types.
- Empty state: "输入关键词搜索消息和客户".

Co-Authored-By: Claude <noreply@anthropic.com>

yunkes/src/layouts/MainLayout.vue

@@ -143,6 +143,10 @@ onMounted(async () => {
           <el-icon><Refresh /></el-icon>
           <span>同步</span>
         </el-menu-item>
+        <el-menu-item index="/search">
+          <el-icon><Search /></el-icon>
+          <span>搜索</span>
+        </el-menu-item>
       </el-menu>
     </el-header>