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/Domain/Registrations/HerregistratieRule.cs

@@ -0,0 +1,32 @@
+namespace BigRegister.Domain.Registrations;
+
+/// <summary>
+/// SERVER-OWNED business rule (ported from the frontend reference impl
+/// registration.policy.ts:isHerregistratieEligible). A registration may apply for
+/// herregistratie only while active and within the window before its deadline.
+/// The endpoint ships the result as a decision flag + reason; the frontend renders it.
+/// </summary>
+public static class HerregistratieRule
+{
+    public const int WindowMonths = 12;
+
+    public static DateOnly? Deadline(Registration reg) =>
+        reg.Status.Tag == StatusTag.Geregistreerd ? reg.Status.HerregistratieDatum : null;
+
+    public static (bool Eligible, string? Reason) Evaluate(
+        Registration reg, DateOnly today, int windowMonths = WindowMonths)
+    {
+        var deadline = Deadline(reg);
+        if (deadline is null)
+            return (false, "Geen actieve registratie.");
+
+        var windowStart = deadline.Value.AddMonths(-windowMonths);
+        return today >= windowStart
+            ? (true, $"Registratie verloopt binnen {windowMonths} maanden ({deadline:yyyy-MM-dd}).")
+            : (false, $"Herregistratie kan vanaf {windowStart:yyyy-MM-dd}.");
+    }
+
+    /// <summary>Invariant: a non-active status must not carry a herregistratie date.</summary>
+    public static bool IsStatusConsistent(RegistrationStatus s) =>
+        s.Tag != StatusTag.Geregistreerd || s.HerregistratieDatum is not null;
+}