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/Submissions/SubmissionRules.cs

@@ -0,0 +1,23 @@
+namespace BigRegister.Domain.Submissions;
+
+/// <summary>
+/// SERVER-OWNED submit rules (ported from the frontend submit-*.ts commands, where
+/// they were hardcoded). Each method returns a rejection reason, or null when the
+/// submission is accepted. The reference is generated server-side on acceptance.
+/// </summary>
+public static class SubmissionRules
+{
+    // RULE: a manually entered diploma cannot be auto-verified.
+    public static string? RejectRegistratie(string diplomaHerkomst) =>
+        diplomaHerkomst == "handmatig"
+            ? "Een handmatig ingevoerd diploma kan niet automatisch worden geverifieerd. Uw aanvraag is doorgestuurd voor handmatige beoordeling."
+            : null;
+
+    // RULE: an application reporting zero worked hours is rejected.
+    public static string? RejectZeroUren(int uren) =>
+        uren == 0 ? "Aanvraag afgewezen: geen gewerkte uren geregistreerd." : null;
+
+    public static string NewReference() =>
+        // ponytail: random reference is fine for a demo; a real system reserves it transactionally.
+        "BIG-2026-" + Random.Shared.Next(100_000, 1_000_000);
+}