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>

.dockerignore

@@ -0,0 +1,12 @@
+# WP-30: backend/Dockerfile does `COPY . .` from the repo root (needs public/letter.css as a
+# sibling of backend/) — without this, that copy would drag in node_modules (~750MB), .git,
+# and every build/output directory, defeating the point of a "lean" image.
+.git
+node_modules
+dist
+storybook-static
+backend/**/bin
+backend/**/obj
+backend/**/bigregister.db*
+backend/openzaak
+documentation.json