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>

backend/README.md

@@ -0,0 +1,102 @@
+# BIG-register BFF (ASP.NET Core)
+
+The backend that hosts the **business rules** for the BIG-register portal. The
+frontend renders the decisions this service computes; it does not recompute them
+(BFF-lite + decision DTOs — see `../docs/architecture/0001-bff-lite-decision-dtos.md`).
+
+No database, no real BRP/DUO: data is in-memory and seeded (`Data/SeedData.cs`),
+but the endpoints, DTOs, status codes and error envelope are production-shaped.
+
+## Run
+
+### Everything (docker-compose, from repo root)
+
+```bash
+docker compose up
+```
+
+- App: <http://localhost:4200>
+- Swagger UI: <http://localhost:5000/swagger>
+
+### Backend only (local)
+
+```bash
+cd backend
+dotnet run --project src/BigRegister.Api
+# → http://localhost:5000/swagger
+```
+
+### Frontend against a local backend
+
+```bash
+npm start          # ng serve, proxies /api → http://localhost:5000 (proxy.conf.json)
+```
+
+### Tests
+
+```bash
+cd backend && dotnet test      # rule unit tests + endpoint integration tests
+```
+
+## API
+
+| Method | Route | Purpose |
+|---|---|---|
+| GET | `/api/dashboard-view` | registration + person + computed herregistratie decision |
+| GET | `/api/notes` | specialisms / aantekeningen |
+| GET | `/api/brp/address` | BRP address lookup (`gevonden:false` = no address) |
+| GET | `/api/duo/diplomas` | diplomas with derived profession + applicable policy questions, + manual fallback |
+| GET | `/api/intake/policy` | scholing threshold (config value) |
+| POST | `/api/registrations` | submit registration → reference, or 422 (manual diploma) |
+| POST | `/api/herregistraties` | submit re-registration → reference, or 422 (0 hours) |
+| POST | `/api/intakes` | submit intake → reference, or 422 (0 hours) |
+
+Rejections use **ProblemDetails (RFC 7807)** with status **422**.
+
+## Where the rules live (`src/BigRegister.Api/Domain/`)
+
+- `Diplomas/DiplomaRules.cs` — profession derivation + which policy questions apply.
+- `Registrations/HerregistratieRule.cs` — eligibility + reason + status invariant.
+- `Intake/IntakePolicy.cs` — scholing threshold.
+- `Submissions/SubmissionRules.cs` — submit rejections + reference generation.
+
+## Typed client (NSwag)
+
+The frontend calls this API through a generated TypeScript client. Regenerate it
+from the contract after a **shape** change:
+
+```bash
+npm run gen:api    # builds backend → swagger.json → src/app/shared/infrastructure/api-client.ts
+```
+
+## Maintainability: changing a policy is one backend change
+
+**Goal:** require every *Verpleegkundige* diploma to confirm a Dutch skills
+assessment. This is a new policy question on a diploma type.
+
+Edit **one file** — `Domain/Diplomas/DiplomaRules.cs`:
+
+```diff
+ public static IReadOnlyList<PolicyQuestion> QuestionsFor(Diploma d)
+ {
+     var questions = new List<PolicyQuestion>();
+     if (d.Engelstalig)
+         questions.Add(NlTaalEngelstalig);
++    if (d.Opleiding == "verpleegkunde")
++        questions.Add(new PolicyQuestion(
++            "bekwaamheid",
++            "Heeft u in de afgelopen vijf jaar een bekwaamheidstoets afgelegd?",
++            QuestionType.JaNee));
+     return questions;
+ }
+```
+
+Rebuild the backend (`docker compose up` or `dotnet run`). The new question now
+appears in the registration wizard for HBO-Verpleegkunde.
+
+- **No frontend change.** The FE renders whatever questions the API returns.
+- **No client regeneration.** The wire shape (`PolicyQuestionDto`) is unchanged —
+  only the data behind it. `npm run gen:api` is only needed when a DTO *shape* changes.
+
+Add a unit test for the new rule in `tests/BigRegister.Tests/RuleTests.cs` and
+you're done.