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.
+"""

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.
+"""

backend/app/platform/__init__.py

@@ -0,0 +1,5 @@
+"""platform layer — cross-cutting primitives (config, db, deps).
+
+The bottom layer: imports nothing from app.domains or app.main. Owns the schema
+lifecycle and request wiring, never business rules (SD-0001 §6.2).
+"""