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/Diplomas/Diploma.cs

@@ -0,0 +1,22 @@
+namespace BigRegister.Domain.Diplomas;
+
+public enum QuestionType
+{
+    JaNee,
+    Tekst,
+}
+
+public sealed record PolicyQuestion(string Id, string Vraag, QuestionType Type);
+
+/// <summary>
+/// A diploma as DUO knows it. The profession and the applicable policy questions
+/// are NOT stored on the diploma — they are DERIVED by the rules below from its
+/// attributes (<see cref="Opleiding"/>, <see cref="Engelstalig"/>).
+/// </summary>
+public sealed record Diploma(
+    string Id,
+    string Naam,
+    string Instelling,
+    int Jaar,
+    string Opleiding,
+    bool Engelstalig);

backend/src/BigRegister.Api/Domain/Diplomas/DiplomaRules.cs

@@ -0,0 +1,68 @@
+namespace BigRegister.Domain.Diplomas;
+
+/// <summary>
+/// SERVER-OWNED business rules for diplomas. This is the single place a policy
+/// changes: which profession a study program maps to, and which policy questions
+/// (geldigheidsvragen) apply to a diploma. The frontend renders these; it never
+/// derives them.
+/// </summary>
+public static class DiplomaRules
+{
+    // RULE: study program → BIG profession.
+    private static readonly Dictionary<string, string> ProfessionByProgram = new(StringComparer.OrdinalIgnoreCase)
+    {
+        ["geneeskunde"] = "Arts",
+        ["verpleegkunde"] = "Verpleegkundige",
+        ["fysiotherapie"] = "Fysiotherapeut",
+        ["farmacie"] = "Apotheker",
+        ["tandheelkunde"] = "Tandarts",
+    };
+
+    public static string ProfessionFor(Diploma d) =>
+        ProfessionByProgram.TryGetValue(d.Opleiding, out var beroep) ? beroep : "Onbekend";
+
+    /// <summary>Professions a user may declare for a manual (unlisted) diploma.</summary>
+    public static IReadOnlyList<string> ManualProfessions() =>
+        ProfessionByProgram.Values.Distinct().ToList();
+
+    // --- Policy questions (geldigheidsvragen) ---
+
+    private static readonly PolicyQuestion NlTaalEngelstalig = new(
+        "nl-taalvaardigheid",
+        "Uw opleiding was Engelstalig. Beheerst u de Nederlandse taal op het vereiste niveau (B2)?",
+        QuestionType.JaNee);
+
+    private static readonly PolicyQuestion NlTaalManual = new(
+        "nl-taalvaardigheid",
+        "Beheerst u de Nederlandse taal op het vereiste niveau (B2)?",
+        QuestionType.JaNee);
+
+    private static readonly PolicyQuestion DiplomaErkend = new(
+        "diploma-erkend",
+        "Is uw diploma erkend door de Nederlandse overheid (bijv. via Nuffic)?",
+        QuestionType.JaNee);
+
+    private static readonly PolicyQuestion Toelichting = new(
+        "toelichting",
+        "Geef een korte toelichting op uw diploma en opleiding.",
+        QuestionType.Tekst);
+
+    /// <summary>
+    /// RULE: an English-language diploma requires proof of Dutch proficiency (B2).
+    /// Add a question here to apply it to a (set of) diploma(s) — a single backend
+    /// change, no frontend change.
+    /// </summary>
+    public static IReadOnlyList<PolicyQuestion> QuestionsFor(Diploma d)
+    {
+        var questions = new List<PolicyQuestion>();
+        if (d.Engelstalig)
+            questions.Add(NlTaalEngelstalig);
+        return questions;
+    }
+
+    /// <summary>
+    /// RULE: a manual diploma is unverified, so the strictest (maximal) set applies.
+    /// </summary>
+    public static IReadOnlyList<PolicyQuestion> ManualQuestions() =>
+        new[] { NlTaalManual, DiplomaErkend, Toelichting };
+}

backend/src/BigRegister.Api/Domain/Intake/IntakePolicy.cs

@@ -0,0 +1,11 @@
+namespace BigRegister.Domain.Intake;
+
+/// <summary>
+/// SERVER-OWNED config value. Below this many NL work-hours the scholing question
+/// is required. The frontend receives this value and applies it for instant UX
+/// feedback, but the backend re-validates on submit as the authority.
+/// </summary>
+public static class IntakePolicy
+{
+    public const int ScholingThreshold = 1000;
+}

backend/src/BigRegister.Api/Domain/People/Person.cs

@@ -0,0 +1,5 @@
+namespace BigRegister.Domain.People;
+
+public sealed record Adres(string Straat, string Postcode, string Woonplaats);
+
+public sealed record Person(string Naam, DateOnly Geboortedatum, Adres Adres);

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;
+}

backend/src/BigRegister.Api/Domain/Registrations/Registration.cs

@@ -0,0 +1,28 @@
+namespace BigRegister.Domain.Registrations;
+
+/// <summary>The three states a BIG registration can be in.</summary>
+public enum StatusTag
+{
+    Geregistreerd,
+    Geschorst,
+    Doorgehaald,
+}
+
+/// <summary>
+/// Status as a flat record: only <see cref="StatusTag.Geregistreerd"/> carries a
+/// herregistratie deadline. The frontend mirrors this as a discriminated union.
+/// </summary>
+public sealed record RegistrationStatus(
+    StatusTag Tag,
+    DateOnly? HerregistratieDatum = null,
+    DateOnly? GeschorstTot = null,
+    string? Reden = null,
+    DateOnly? DoorgehaaldOp = null);
+
+public sealed record Registration(
+    string BigNummer,
+    string Naam,
+    string Beroep,
+    DateOnly Registratiedatum,
+    DateOnly Geboortedatum,
+    RegistrationStatus Status);

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);
+}