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/.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