feat: implement per-user chat clearing by filtering messages based on new clearedAt timestamps

prisma/schema.prisma

@@ -503,6 +503,10 @@ model Conversation {
   userFavorited    Boolean   @default(false)
   agentFavorited   Boolean   @default(false)
 
+  // Clear chat (per-user) — messages before this timestamp are hidden for that user
+  userClearedAt    DateTime?
+  agentClearedAt   DateTime?
+
   // Timestamps
   createdAt        DateTime  @default(now())
   updatedAt        DateTime  @updatedAt