add build development phases

dev/shared-working-context/backend/STATUS.md

@@ -0,0 +1,15 @@
+# Backend status log (append-only)
+
+One block per completed backend phase. Newest at the top. Backend lane writes here; frontend reads.
+
+<!-- TEMPLATE — copy for each phase
+## backend-phase-N — <Title> — <YYYY-MM-DD>
+- **Shipped:** <entities/endpoints/seams in one or two lines>
+- **Contracts:** dev/contracts/domains/<domain>.md + openapi snapshot refreshed (yes/no)
+- **Mocked:** <which seams> (see reports/mocks-registry.md)
+- **Gate:** build clean / tests green
+- **Handoff:** backend/handoff/after-backend-phase-N.md
+- **Notes for frontend:** <anything load-bearing>
+-->
+
+_(no phases completed yet)_