feat(api): add conversation archive with vector search (MS22-DB-004, MS22-API-004) (#587)

Co-authored-by: Jason Woltje <jason@diversecanvas.com>
Co-committed-by: Jason Woltje <jason@diversecanvas.com>

apps/api/prisma/schema.prisma

@@ -314,6 +314,7 @@ model Workspace {
   llmUsageLogs                 LlmUsageLog[]
   userCredentials              UserCredential[]
   terminalSessions             TerminalSession[]
+  conversationArchives         ConversationArchive[]
 
   @@index([ownerId])
   @@map("workspaces")
@@ -1593,3 +1594,33 @@ model TerminalSession {
   @@index([workspaceId, status])
   @@map("terminal_sessions")
 }
+
+// ============================================
+// CONVERSATION ARCHIVE MODULE
+// ============================================
+
+model ConversationArchive {
+  id           String                       @id @default(uuid()) @db.Uuid
+  workspaceId  String                       @map("workspace_id") @db.Uuid
+  sessionId    String                       @map("session_id")
+  agentId      String                       @map("agent_id")
+  messages     Json
+  messageCount Int                          @map("message_count")
+  summary      String                       @db.Text
+  // Note: vector dimension (1536) must match EMBEDDING_DIMENSION constant in @mosaic/shared
+  embedding    Unsupported("vector(1536)")?
+  startedAt    DateTime                     @map("started_at") @db.Timestamptz
+  endedAt      DateTime?                    @map("ended_at") @db.Timestamptz
+  metadata     Json                         @default("{}")
+  createdAt    DateTime                     @default(now()) @map("created_at") @db.Timestamptz
+  updatedAt    DateTime                     @updatedAt @map("updated_at") @db.Timestamptz
+
+  // Relations
+  workspace Workspace @relation(fields: [workspaceId], references: [id], onDelete: Cascade)
+
+  @@unique([workspaceId, sessionId])
+  @@index([workspaceId])
+  @@index([agentId])
+  @@index([startedAt])
+  @@map("conversation_archives")
+}