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/migrations/20260228000000_ms22_conversation_archive/migration.sql

@@ -0,0 +1,33 @@
+-- CreateTable
+CREATE TABLE "conversation_archives" (
+    "id" UUID NOT NULL,
+    "workspace_id" UUID NOT NULL,
+    "session_id" TEXT NOT NULL,
+    "agent_id" TEXT NOT NULL,
+    "messages" JSONB NOT NULL,
+    "message_count" INTEGER NOT NULL,
+    "summary" TEXT NOT NULL,
+    "embedding" vector(1536),
+    "started_at" TIMESTAMPTZ NOT NULL,
+    "ended_at" TIMESTAMPTZ,
+    "metadata" JSONB NOT NULL DEFAULT '{}',
+    "created_at" TIMESTAMPTZ NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    "updated_at" TIMESTAMPTZ NOT NULL,
+
+    CONSTRAINT "conversation_archives_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateIndex
+CREATE UNIQUE INDEX "conversation_archives_workspace_id_session_id_key" ON "conversation_archives"("workspace_id", "session_id");
+
+-- CreateIndex
+CREATE INDEX "conversation_archives_workspace_id_idx" ON "conversation_archives"("workspace_id");
+
+-- CreateIndex
+CREATE INDEX "conversation_archives_agent_id_idx" ON "conversation_archives"("agent_id");
+
+-- CreateIndex
+CREATE INDEX "conversation_archives_started_at_idx" ON "conversation_archives"("started_at");
+
+-- AddForeignKey
+ALTER TABLE "conversation_archives" ADD CONSTRAINT "conversation_archives_workspace_id_fkey" FOREIGN KEY ("workspace_id") REFERENCES "workspaces"("id") ON DELETE CASCADE ON UPDATE CASCADE;

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")
+}