feat(slice-1): backend skeleton, deps, and import-linter layer contract

Four-layer package (main > domains > platform) per SD-0001 §6.2; GraphQL api layer
deferred. main.py is a stub so the top layer exists from the start (fleshed out in
the /healthz task). lint-imports green on the skeleton.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/app/main.py

@@ -0,0 +1,5 @@
+"""ecomm backend — FastAPI app factory + the REST BFF.
+
+Stub in SLICE-1 Task 1 so the top layer of the import-linter contract exists from the
+start; the FastAPI factory and /healthz are added in Task 5.
+"""