feat: implement per-user soft delete for conversations with automatic restoration on new messages

prisma/schema.prisma

@@ -507,6 +507,10 @@ model Conversation {
   userClearedAt    DateTime?
   agentClearedAt   DateTime?
 
+  // Soft delete (per-user) — conversation hidden for that user until a new message arrives
+  userDeletedAt    DateTime?
+  agentDeletedAt   DateTime?
+
   // Timestamps
   createdAt        DateTime  @default(now())
   updatedAt        DateTime  @updatedAt