feat: 实现 AI 智能搜索功能

添加语义搜索能力，支持自然语言查询找到相关项目。

- 数据库：新增 embedding 字段用于向量存储
- 前端：新增 AI 搜索栏和结果组件，支持传统/AI 模式切换
- API：新增 /api/search/ai 端点处理语义搜索请求
- 国际化：添加 AI 搜索相关中英文翻译
- 探索任务：允许 FAILED 状态直接转到 IN_PROGRESS 简化重试

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

src/messages/en.json

@@ -1,6 +1,7 @@
 {
   "common": {
     "search": "Search",
+    "aiSearch": "AI Search",
     "loading": "Loading...",
     "noResults": "No results found",
     "noProjects": "No projects yet",
@@ -21,6 +22,7 @@
     "featuredProjects": "Featured Projects",
     "browseByTag": "Browse by Tag",
     "searchPlaceholder": "Search AI projects...",
+    "aiSearchPlaceholder": "Describe what you're looking for, e.g.: AI tools that can generate videos...",
     "metaTitle": "Agent Park - AI Project Navigator",
     "metaDescription": "Discover and explore quality AI projects from across the web",
     "heroTitle": "AI PROJECT",