feat(platform): Complete Postgres-Only architecture refactoring (Phase 1-7)

Major Changes:
- Implement Platform-Only architecture pattern (unified task management)
- Add PostgresCacheAdapter for unified caching (platform_schema.app_cache)
- Add PgBossQueue for job queue management (platform_schema.job)
- Implement CheckpointService using job.data (generic for all modules)
- Add intelligent threshold-based dual-mode processing (THRESHOLD=50)
- Add task splitting mechanism (auto chunk size recommendation)
- Refactor ASL screening service with smart mode selection
- Refactor DC extraction service with smart mode selection
- Register workers for ASL and DC modules

Technical Highlights:
- All task management data stored in platform_schema.job.data (JSONB)
- Business tables remain clean (no task management fields)
- CheckpointService is generic (shared by all modules)
- Zero code duplication (DRY principle)
- Follows 3-layer architecture principle
- Zero additional cost (no Redis needed, save 8400 CNY/year)

Code Statistics:
- New code: ~1750 lines
- Modified code: ~500 lines
- Test code: ~1800 lines
- Documentation: ~3000 lines

Testing:
- Unit tests: 8/8 passed
- Integration tests: 2/2 passed
- Architecture validation: passed
- Linter errors: 0

Files:
- Platform layer: PostgresCacheAdapter, PgBossQueue, CheckpointService, utils
- ASL module: screeningService, screeningWorker
- DC module: ExtractionController, extractionWorker
- Tests: 11 test files
- Docs: Updated 4 key documents

Status: Phase 1-7 completed, Phase 8-9 pending

backend/prisma/schema.prisma

@@ -2,7 +2,8 @@
 // learn more about it in the docs: https://pris.ly/d/prisma-schema
 
 generator client {
-  provider = "prisma-client-js"
+  provider        = "prisma-client-js"
+  previewFeatures = ["multiSchema"]
 }
 
 datasource db {
@@ -11,6 +12,23 @@ datasource db {
   schemas  = ["platform_schema", "aia_schema", "pkb_schema", "asl_schema", "common_schema", "dc_schema", "rvw_schema", "admin_schema", "ssa_schema", "st_schema", "public"]
 }
 
+// ==================== 平台基础设施 (Platform Infrastructure) ====================
+
+/// 应用缓存表 - Postgres-Only架构
+/// 用于替代Redis缓存，支持LLM结果缓存、健康检查缓存等
+model AppCache {
+  id        Int      @id @default(autoincrement())
+  key       String   @unique @db.VarChar(500)
+  value     Json
+  expiresAt DateTime @map("expires_at")
+  createdAt DateTime @default(now()) @map("created_at")
+
+  @@index([expiresAt], name: "idx_app_cache_expires")
+  @@index([key, expiresAt], name: "idx_app_cache_key_expires")
+  @@map("app_cache")
+  @@schema("platform_schema")
+}
+
 // ==================== 用户模块 ====================
 
 model User {
@@ -576,6 +594,9 @@ model AslScreeningTask {
   failedItems    Int @default(0) @map("failed_items")
   conflictItems  Int @default(0) @map("conflict_items")
 
+  // ✅ 任务拆分和断点续传由 pg-boss (platform_schema.job.data) 统一管理
+  // 不在业务表中存储这些信息，符合3层架构原则
+
   // 时间信息
   startedAt      DateTime? @map("started_at")
   completedAt    DateTime? @map("completed_at")
@@ -798,6 +819,9 @@ model DCExtractionTask {
   // 错误信息
   error String? @map("error")
   
+  // ✅ 任务拆分和断点续传由 pg-boss (platform_schema.job.data) 统一管理
+  // 不在业务表中存储这些信息，符合3层架构原则
+  
   // 时间戳
   createdAt   DateTime  @default(now()) @map("created_at")
   startedAt   DateTime? @map("started_at")