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>

.gitignore

@@ -6,3 +6,6 @@ node_modules/
 dist/
 .env
 *.local
+.pytest_cache/
+frontend/dist/
+backend/var/

backend/.importlinter

@@ -0,0 +1,14 @@
+# Enforces SD-0001 §6.2: the four-layer contract main > domains > platform, with
+# imports flowing only downward. The GraphQL `api` layer is deferred (§2), so the
+# MVP contract has three layers; `app.api` is added back between main and domains
+# when GraphQL arrives. Run from backend/:  .venv/bin/lint-imports
+[importlinter]
+root_package = app
+
+[importlinter:contract:layers]
+name = App layering (main > domains > platform)
+type = layers
+layers =
+    app.main
+    app.domains
+    app.platform

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).
+"""

backend/pytest.ini

@@ -0,0 +1,7 @@
+[pytest]
+testpaths = tests
+python_files = test_*.py
+addopts = -q
+# Put backend/ (this rootdir) on sys.path so tests import the `app` package under
+# pytest's default prepend import mode (tests/ has no __init__.py).
+pythonpath = .

backend/requirements.txt

@@ -0,0 +1,7 @@
+fastapi>=0.110
+uvicorn[standard]>=0.29
+httpx>=0.27
+psycopg[binary]>=3.1
+psycopg-pool>=3.2
+pytest>=8.0
+import-linter>=2.0