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>

prisma/schema.prisma

@@ -39,23 +39,25 @@ enum TaskStatus {
 // ================================
 
 model Project {
-  id             String        @id @default(cuid())
-  name           String
-  nameEn         String?
-  slug           String        @unique
-  description    String
-  descriptionEn  String?
-  content        String?       @db.Text
-  contentEn      String?       @db.Text
-  status         ProjectStatus @default(ACTIVE)
-  source         String?
-  createdAt      DateTime      @default(now())
-  updatedAt      DateTime      @updatedAt
+  id                String        @id @default(cuid())
+  name              String
+  nameEn            String?
+  slug              String        @unique
+  description       String
+  descriptionEn     String?
+  content           String?       @db.Text
+  contentEn         String?       @db.Text
+  status            ProjectStatus @default(ACTIVE)
+  source            String?
+  embedding         Unsupported("vector(1536)")?
+  embeddingUpdatedAt DateTime?
+  createdAt         DateTime      @default(now())
+  updatedAt         DateTime      @updatedAt
 
   // Relations
-  tags           ProjectTag[]
-  links          ExternalLink[]
-  discoveryTasks ProjectDiscoveryTask[]
+  tags              ProjectTag[]
+  links             ExternalLink[]
+  discoveryTasks    ProjectDiscoveryTask[]
 
   // Indexes
   @@index([status, createdAt], map: "idx_project_status_createdAt")