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>

docs/scratchpads/ms22-conversation-archive.md

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+# MS22 — Conversation Archive Module
+
+## Objective
+
+Implement ConversationArchive module: ingest OpenClaw session logs, store with vector embeddings for semantic search.
+
+## Deliverables
+
+1. ConversationArchive Prisma model
+2. NestJS module at apps/api/src/conversation-archive/
+3. Endpoints: ingest, search, list, get-by-id
+4. Register in app.module.ts
+5. Migrate, lint, build, commit
+
+## Plan
+
+- Add model to schema.prisma (end of file)
+- Add relation to Workspace model
+- Create module structure: dto/, service, controller, spec, module
+- Use EmbeddingService from knowledge module (import KnowledgeModule or just PrismaModule + embed inline)
+- Follow pattern: AuthGuard + WorkspaceGuard + PermissionGuard
+- Endpoint prefix: conversations (maps to /api/conversations)
+- Vector search: $queryRaw with <=> operator (cosine distance)
+
+## Assumptions
+
+- ASSUMPTION: Embedding is stored inline on ConversationArchive (not a separate table) — simpler and sufficient for this use case, matches MemoryEmbedding pattern
+- ASSUMPTION: Import KnowledgeModule to reuse EmbeddingService (it exports it)
+- ASSUMPTION: messageCount computed server-side from messages array length on ingest
+- ASSUMPTION: Permission level WORKSPACE_MEMBER for ingest/search, WORKSPACE_ANY for list/get
+
+## Progress
+
+- [ ] Schema model
+- [ ] Migration
+- [ ] DTOs
+- [ ] Service
+- [ ] Controller
+- [ ] Spec
+- [ ] Module
+- [ ] app.module.ts registration
+- [ ] Lint + build
+- [ ] Commit
+
+## Risks
+
+- EmbeddingService exports from knowledge.module — need to import KnowledgeModule
+- Migration requires live DB (may need --skip-generate flag if no DB access)