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>

src/app/registratie/application/big-profile.store.ts

@@ -42,9 +42,10 @@ export class BigProfileStore {
   readonly decisions = computed<RemoteData<Err, HerregistratieDecisions>>(() => map(this.view(), (v) => v.decisions));
 
   /** Specialisms/notes stay a separate stream (they have their own empty state). */
-  readonly aantekeningen = computed<RemoteData<Err, Aantekening[]>>(() =>
-    fromResource(this.aantekeningenRes, (v) => v.length === 0),
-  );
+  readonly aantekeningen = computed<RemoteData<Err, Aantekening[]>>(() => {
+    const rd = fromResource(this.aantekeningenRes, (v) => !v || v.length === 0);
+    return rd.tag === 'Success' ? { tag: 'Success', value: rd.value ?? [] } : rd;
+  });
 
   // --- Optimistic herregistratie state, shared with the dashboard -----------
   private pending = signal(false);