Add reviewer, artifact store, and paginated queries

docs/backlog/01_execution_backlog.md

@@ -20,7 +20,7 @@
 
 - [x] graph scheduler service
 - [x] node runner abstraction
-- [ ] reviewer contract
+- [x] reviewer contract
 - [x] finalizer contract
 - [x] resume/cancel use cases
 - [x] idempotency policy
@@ -31,8 +31,8 @@
 - [x] SQLAlchemy models
 - [x] repositories for PostgreSQL/SQLite
 - [x] migrations
-- [ ] artifact metadata persistence
-- [ ] event store queries with pagination
+- [x] artifact metadata persistence
+- [x] event store queries with pagination
 
 ## Package D: Integrations
 
@@ -53,7 +53,7 @@
 
 ## Package F: Validation
 
-- [ ] contract tests
-- [ ] scenario tests
-- [ ] smoke suite
+- [x] contract tests
+- [x] scenario tests
+- [x] smoke suite
 - [ ] static checks in CI