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/src/conversation-archive/dto/search-conversation.dto.ts

@@ -0,0 +1,40 @@
+import {
+  IsString,
+  IsOptional,
+  MinLength,
+  MaxLength,
+  IsInt,
+  Min,
+  Max,
+  IsNumber,
+} from "class-validator";
+import { Type } from "class-transformer";
+
+/**
+ * DTO for semantic search across conversation archives
+ */
+export class SearchConversationDto {
+  @IsString()
+  @MinLength(1)
+  @MaxLength(1000)
+  query!: string;
+
+  @IsOptional()
+  @IsString()
+  @MaxLength(500)
+  agentId?: string;
+
+  @IsOptional()
+  @Type(() => Number)
+  @IsInt()
+  @Min(1)
+  @Max(100)
+  limit?: number;
+
+  @IsOptional()
+  @Type(() => Number)
+  @IsNumber()
+  @Min(0)
+  @Max(1)
+  similarityThreshold?: number;
+}