fix(aia,ssa,asl,infra): harden SSE transport and stabilize attachment context

Deliver SSE protocol hardening for SAE/HTTP2 paths, add graceful shutdown health behavior, and improve SSA retry UX for transient stream failures. For AIA, persist attachment extraction results in database with cache read-through fallback, plus production cache safety guard to prevent memory-cache drift in multi-instance deployments; also restore ASL SR page scrolling behavior.

Made-with: Cursor

backend/prisma/schema.prisma

@@ -127,6 +127,31 @@ model Message {
   @@schema("aia_schema")
 }
 
+/// AIA 附件持久化（数据库真相源，缓存仅加速）
+model AiaAttachment {
+  id             String   @id
+  userId         String   @map("user_id")
+  conversationId String   @map("conversation_id")
+  filename       String
+  mimeType       String?  @map("mime_type")
+  size           Int
+  ossUrl         String   @map("oss_url")
+  textContent    String?  @map("text_content") @db.Text
+  extractStatus  String   @default("success") @map("extract_status") // success | failed | empty
+  extractError   String?  @map("extract_error")
+  tokenCount     Int      @default(0) @map("token_count")
+  truncated      Boolean  @default(false)
+  createdAt      DateTime @default(now()) @map("created_at")
+  updatedAt      DateTime @updatedAt @map("updated_at")
+
+  @@index([conversationId], map: "idx_aia_attachments_conversation_id")
+  @@index([userId], map: "idx_aia_attachments_user_id")
+  @@index([extractStatus], map: "idx_aia_attachments_extract_status")
+  @@index([createdAt], map: "idx_aia_attachments_created_at")
+  @@map("attachments")
+  @@schema("aia_schema")
+}
+
 model KnowledgeBase {
   id             String      @id @default(uuid())
   userId         String      @map("user_id")