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/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