feat(asl): Add DeepSearch smart literature retrieval MVP

Features:
- Integrate unifuncs DeepSearch API (OpenAI compatible protocol)
- SSE real-time streaming for AI thinking process display
- Natural language input, auto-generate PubMed search strategy
- Extract and display PubMed literature links
- Database storage for task records (asl_research_tasks)

Backend:
- researchService.ts - Core business logic with SSE streaming
- researchController.ts - SSE stream endpoint
- researchWorker.ts - Async task worker (backup mode)
- schema.prisma - AslResearchTask model

Frontend:
- ResearchSearch.tsx - Search page with unified content stream
- ResearchSearch.css - Styling (unifuncs-inspired simple design)
- ASLLayout.tsx - Enable menu item
- api/index.ts - Add research API functions

API Endpoints:
- POST /api/v1/asl/research/stream - SSE streaming search
- POST /api/v1/asl/research/tasks - Async task creation
- GET /api/v1/asl/research/tasks/:taskId/status - Task status

Documentation:
- Development record for DeepSearch integration
- Update ASL module status (v1.5)
- Update system status (v3.7)

Known limitations:
- SSE mode, task interrupts when leaving page
- Cost ~0.3 RMB per search (unifuncs API)

backend/prisma/schema.prisma

@@ -476,6 +476,50 @@ model AslFulltextScreeningTask {
   @@schema("asl_schema")
 }
 
+/// 智能文献检索任务（DeepSearch）
+model AslResearchTask {
+  id               String   @id @default(uuid())
+  
+  // 关联
+  projectId        String   @map("project_id")
+  userId           String   @map("user_id")
+  
+  // 检索输入
+  query            String                           // 用户的自然语言查询
+  filters          Json?                            // 🔜 后续：高级筛选 { yearFrom, yearTo, articleTypes }
+  
+  // unifuncs 任务
+  externalTaskId   String?  @map("external_task_id")
+  
+  // 状态
+  status           String   @default("pending")     // pending/processing/completed/failed
+  errorMessage     String?  @map("error_message")
+  
+  // 结果
+  resultCount      Int?     @map("result_count")
+  rawResult        String?  @map("raw_result") @db.Text
+  reasoningContent String?  @map("reasoning_content") @db.Text  // AI思考过程
+  literatures      Json?                            // 解析后的文献列表
+  
+  // 统计（🔜 后续展示）
+  tokenUsage       Json?    @map("token_usage")
+  searchCount      Int?     @map("search_count")
+  readCount        Int?     @map("read_count")
+  iterations       Int?
+  
+  // 时间
+  createdAt        DateTime @default(now()) @map("created_at")
+  updatedAt        DateTime @updatedAt @map("updated_at")
+  completedAt      DateTime? @map("completed_at")
+
+  @@index([projectId], map: "idx_research_tasks_project_id")
+  @@index([userId], map: "idx_research_tasks_user_id")
+  @@index([status], map: "idx_research_tasks_status")
+  @@index([createdAt], map: "idx_research_tasks_created_at")
+  @@map("research_tasks")
+  @@schema("asl_schema")
+}
+
 model AslFulltextScreeningResult {
   id                  String                   @id @default(uuid())
   taskId              String                   @map("task_id")