feat(storage): integrate Alibaba Cloud OSS for file persistence - Add OSSAdapter and LocalAdapter with StorageFactory pattern - Integrate PKB module with OSS upload - Rename difyDocumentId to storageKey - Create 4 OSS buckets and development specification

backend/prisma/schema.prisma

@@ -152,7 +152,7 @@ model Document {
   fileType          String        @map("file_type")
   fileSizeBytes     BigInt        @map("file_size_bytes")
   fileUrl           String        @map("file_url")
-  difyDocumentId    String        @map("dify_document_id")
+  storageKey        String        @map("dify_document_id")  // 原 difyDocumentId，现存储 OSS 路径
   status            String        @default("uploading")
   progress          Int           @default(0)
   errorMessage      String?       @map("error_message")
@@ -168,7 +168,7 @@ model Document {
   batchResults      BatchResult[] @relation("DocumentBatchResults")
   knowledgeBase     KnowledgeBase @relation("KnowledgeBaseDocuments", fields: [kbId], references: [id], onDelete: Cascade)
 
-  @@index([difyDocumentId], map: "idx_pkb_documents_dify_document_id")
+  @@index([storageKey], map: "idx_pkb_documents_dify_document_id")  // 保留原索引名
   @@index([extractionMethod], map: "idx_pkb_documents_extraction_method")
   @@index([kbId], map: "idx_pkb_documents_kb_id")
   @@index([status], map: "idx_pkb_documents_status")