feat(backend): optional lean deployable image (WP-30 #5)

Multi-stage backend/Dockerfile (sdk build -> aspnet:10.0 runtime, ~312MB) +
docker-compose.prod.yml, additive only — not wired into CI or the existing
dev docker-compose.yml (which keeps the SDK image for dotnet run hot-reload).
New .dockerignore keeps the build context lean (node_modules alone is
~750MB) since the Dockerfile COPYs from the repo root to pick up
public/letter.css (WP-25's FE<->BE letter contract) as a sibling of backend/.

Verified for real: built the image, ran it, and curled a live
GET /api/v1/brief/preview against the running container — got back the
actual rendered letter HTML with letter.css inlined, confirming the
walk-up-from-BaseDirectory lookup resolves inside this image layout too.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docker-compose.prod.yml

@@ -0,0 +1,14 @@
+# WP-30: optional lean deployable backend image — NOT used by the dev demo (docker-compose.yml
+# keeps the SDK image there for `dotnet run` hot-reload; rewriting that one into a prod image
+# would lose the bind-mount hot-reload it's for). This is an additive artifact only, not wired
+# into CI. `docker compose -f docker-compose.prod.yml up --build` → Swagger at :5000/swagger.
+services:
+  api:
+    build:
+      context: .
+      dockerfile: backend/Dockerfile
+    environment:
+      - ASPNETCORE_ENVIRONMENT=Production
+      - ASPNETCORE_URLS=http://+:8080
+    ports:
+      - '5000:8080'