docs: Day 12-13 completion summary and milestone update

backend/prisma/schema.prisma

@@ -78,9 +78,11 @@ model Conversation {
   modelName    String   @default("deepseek-v3") @map("model_name")
   messageCount Int      @default(0) @map("message_count")
   totalTokens  Int      @default(0) @map("total_tokens")
+  metadata     Json?
   
   createdAt    DateTime @default(now()) @map("created_at")
   updatedAt    DateTime @updatedAt @map("updated_at")
+  deletedAt    DateTime? @map("deleted_at")
   
   user         User     @relation(fields: [userId], references: [id], onDelete: Cascade)
   project      Project? @relation(fields: [projectId], references: [id], onDelete: Cascade)
@@ -90,6 +92,7 @@ model Conversation {
   @@index([projectId])
   @@index([agentId])
   @@index([createdAt])
+  @@index([deletedAt])
   @@map("conversations")
 }
 
@@ -98,6 +101,7 @@ model Message {
   conversationId String   @map("conversation_id")
   role           String
   content        String   @db.Text
+  model          String?
   metadata       Json?
   tokens         Int?
   isPinned       Boolean  @default(false) @map("is_pinned")