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/domains/__init__.py

@@ -0,0 +1,6 @@
+"""domains layer — bounded contexts (accounts, storefronts).
+
+Empty in SLICE-1; the accounts domain lands in SLICE-2 and storefronts in SLICE-3.
+The package exists now so the layer contract (.importlinter) has a target and later
+slices add to a stable seam. Domains import only from app.platform, never upward.
+"""