Add ASP.NET Core backend hosting business rules; FE consumes via typed client

Move the authoritative business rules off the frontend into a real backend,
realising the BFF-lite + decision-DTO design (ADR-0001) that until now lived
only in static mock JSON.

Backend (backend/):
- ASP.NET Core (.NET 10) minimal API, contract-first, Swagger UI at /swagger.
- DDD Domain/ rules layer: profession derivation + applicable policy questions
  (DiplomaRules), herregistratie eligibility + reason (HerregistratieRule),
  scholing threshold (IntakePolicy), submit rejections + reference generation
  (SubmissionRules). In-memory seeded data, ProblemDetails (RFC 7807) errors.
- 27 xUnit tests: rule units + endpoint integration incl. BRP no-address and
  DUO not-found fallbacks and 422 submit paths.

Frontend (only infrastructure/ + contracts/ change, as the architecture promised):
- NSwag-generated typed client (api-client.ts), routed through Angular HttpClient
  via a small fetch adapter so the ?scenario= interceptor still applies.
- GET adapters use resource({ loader: client.x }); submit commands call the client
  and map ProblemDetails -> err. The hardcoded uren==0 / manual-diploma rules are
  deleted (now server-side). Domain, stores, UI and format validators unchanged.
- Deleted the now-dead public/mock/*.json.

Tooling/docs:
- npm start proxies /api -> backend; npm run gen:api regenerates the client;
  docker compose up runs both (bind mounts use :z for SELinux/Fedora).
- backend/README.md walkthrough: adding a policy question is a one-file backend
  change, no FE change, no client regen. Updated CLAUDE.md + ARCHITECTURE.md.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docker-compose.yml

@@ -0,0 +1,34 @@
+# ponytail: dev-server images (not multi-stage prod builds) — this is a demo.
+# `docker compose up` → app at http://localhost:4200, Swagger at http://localhost:5000/swagger
+services:
+  api:
+    image: mcr.microsoft.com/dotnet/sdk:10.0
+    working_dir: /src
+    command: dotnet run --project src/BigRegister.Api --urls http://+:5000
+    environment:
+      - ASPNETCORE_ENVIRONMENT=Development
+    volumes:
+      # ':z' relabels for SELinux (Fedora/RHEL); harmless on other hosts.
+      - ./backend:/src:z
+      - api-bin:/src/src/BigRegister.Api/bin
+      - api-obj:/src/src/BigRegister.Api/obj
+    ports:
+      - "5000:5000"
+
+  web:
+    image: node:24
+    working_dir: /app
+    # Uses the committed generated client (no codegen at startup); proxies /api → api container.
+    command: sh -c "npm ci && npx ng serve --host 0.0.0.0 --proxy-config proxy.conf.docker.json"
+    volumes:
+      - ./:/app:z
+      - web-modules:/app/node_modules
+    ports:
+      - "4200:4200"
+    depends_on:
+      - api
+
+volumes:
+  api-bin:
+  api-obj:
+  web-modules: