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

- Add ConversationArchive Prisma model with pgvector(1536) embedding field
- Migration: 20260228000000_ms22_conversation_archive
- NestJS module at apps/api/src/conversation-archive/ with service, controller, DTOs, spec
- POST /api/conversations/ingest — ingest session logs, auto-embed via EmbeddingService
- POST /api/conversations/search — vector similarity search with agentId filter
- GET  /api/conversations — paginated list with agentId + date range filters
- GET  /api/conversations/:id — fetch full conversation including messages
- Register ConversationArchiveModule in app.module.ts
- 8 unit tests, all passing (vitest)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/api/prisma/schema.prisma

@@ -313,6 +313,7 @@ model Workspace {
   llmUsageLogs                 LlmUsageLog[]
   userCredentials              UserCredential[]
   terminalSessions             TerminalSession[]
+  conversationArchives         ConversationArchive[]
 
   @@index([ownerId])
   @@map("workspaces")
@@ -1575,3 +1576,33 @@ 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")
+}