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/src/BigRegister.Api/Contracts/Mappers.cs

@@ -0,0 +1,33 @@
+using BigRegister.Domain.Diplomas;
+using BigRegister.Domain.People;
+using BigRegister.Domain.Registrations;
+
+namespace BigRegister.Api.Contracts;
+
+/// <summary>Domain → wire DTO mapping (the anti-corruption boundary, server side).</summary>
+public static class Mappers
+{
+    private static string D(DateOnly d) => d.ToString("yyyy-MM-dd");
+
+    public static RegistrationStatusDto ToDto(this RegistrationStatus s) => new(
+        Tag: s.Tag.ToString(),
+        HerregistratieDatum: s.HerregistratieDatum is { } h ? D(h) : null,
+        GeschorstTot: s.GeschorstTot is { } g ? D(g) : null,
+        Reden: s.Reden,
+        DoorgehaaldOp: s.DoorgehaaldOp is { } x ? D(x) : null);
+
+    public static RegistrationDto ToDto(this Registration r) => new(
+        r.BigNummer, r.Naam, r.Beroep, D(r.Registratiedatum), D(r.Geboortedatum), r.Status.ToDto());
+
+    public static AdresDto ToDto(this Adres a) => new(a.Straat, a.Postcode, a.Woonplaats);
+
+    public static PersonDto ToDto(this Person p) => new(p.Naam, D(p.Geboortedatum), p.Adres.ToDto());
+
+    public static PolicyQuestionDto ToDto(this PolicyQuestion q) => new(
+        q.Id, q.Vraag, q.Type == QuestionType.JaNee ? "ja-nee" : "tekst");
+
+    public static DuoDiplomaDto ToDto(this Diploma d) => new(
+        d.Id, d.Naam, d.Instelling, d.Jaar,
+        DiplomaRules.ProfessionFor(d),
+        DiplomaRules.QuestionsFor(d).Select(q => q.ToDto()).ToList());
+}