test(backend): add integration tests for all 7 route modules [A5]
- Add test-helpers.ts with buildTestApp() + resetMemoryDatastore()
- notes: 11 tests (CRUD, archive, filter, search, validation)
- workspaces: 7 tests (CRUD, summaries with noteCount, validation)
- note-tasks: 6 tests (CRUD, filter by workspaceId, validation)
- note-artifacts: 7 tests (CRUD, filter by noteId, validation)
- note-relationships: 4 tests (create, list, validation)
- note-agent-actions: 8 tests (CRUD, pending, batch-review, validation)
- saved-views: 8 tests (CRUD, filter by scope, delete, validation)
- Fix listPendingActions to avoid unsupported $in operator in memory provider
- Total: 75 backend tests (was 24)
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