feat(#13): S-12c-1 — behandel BFF auth + werkbak (ADR-0013) (#85)

## What & why

First half of **S-12c** (behandel-portal backend), per **ADR-0013** (decisions recorded in #84):

- **BFF multi-realm auth.** A second JWT bearer scheme (`medewerker`) alongside the default `digid` scheme. On validation it lifts Keycloak's `realm_access.roles` onto the principal, and a `behandelaar` policy (medewerker scheme + `behandelaar` role) gates `/behandel/*`. Self-service keeps the digid scheme.
- **Werkbak = Flowable tasks.** The domain `Werkbak` query reads the open `Beoordelen` tasks (§8.2, S-12b's `IUserTaskClient`) and enriches each with its aggregate's bsn + status; `GET /behandel/werkbak` (domain) is proxied by the BFF `GET /behandel/werkbak` behind the behandelaar policy. The read projection stays the anonymous openbaar model (no premature `IN_BEHANDELING`/personal-data plumbing — deferred in ADR-0008).

Behavior: `/behandel/werkbak` is **401** without a token, **403** for a medewerker lacking the role, **200 + werkbak** for a behandelaar.

**S-12c-2** (next): `POST /behandel/registrations/{id}/decide` → domain decision + complete the Flowable task.

## Definition of Done

- [x] Linked issue: #13 (umbrella, `refs`); closes the adr-proposal #84
- [x] Tests first; red → green per layer
- [x] Unit + acceptance green (`make unit`): domain 78, bff 23, acceptance 9 (+ acl/event-subscriber unaffected)
- [x] api-client `test` green; openapi.json regenerated (drift guard passes)
- [x] Mutation ≥ break(90): **domain 100%, bff 100%**
- [x] ADR-0013 added; `Keycloak__MedewerkerAuthority` wired into compose
- [ ] CI green (pending)

Part of #13. closes #84

Reviewed-on: #85

services/bff/Bff.Api/DownstreamClients.cs

@@ -13,10 +13,17 @@ public sealed record ProjectionEntry(string Id, string Status, string? Reference
 /// <summary>A public-safe openbaar register row — only non-sensitive fields leave the BFF.</summary>
 public sealed record OpenbaarEntry(string Id, string Status, string? Reference);
 
+/// <summary>A behandelaar's werkbak row: a registration awaiting beoordeling, with the bsn + status a
+/// behandelaar sees (staff view — reached only behind medewerker/behandelaar authorization, S-12c).</summary>
+public sealed record WerkbakItem(string RegistrationId, string Bsn, string Status);
+
 /// <summary>Port to the Domain Service (§8.3: the BFF is the portals' only backend; it fans out).</summary>
 public interface IDomainClient
 {
     Task<SubmitAccepted> SubmitRegistrationAsync(string bsn, CancellationToken ct = default);
+
+    /// <summary>The behandelaar's werkbak — registrations awaiting beoordeling.</summary>
+    Task<IReadOnlyList<WerkbakItem>> GetWerkbakAsync(CancellationToken ct = default);
 }
 
 /// <summary>Port to the read projection.</summary>
@@ -37,6 +44,9 @@ public sealed class DomainClient(HttpClient http) : IDomainClient
         return new SubmitAccepted(dto.RegistrationId, dto.Status);
     }
 
+    public async Task<IReadOnlyList<WerkbakItem>> GetWerkbakAsync(CancellationToken ct = default)
+        => await http.GetFromJsonAsync<List<WerkbakItem>>("behandel/werkbak", ct) ?? [];
+
     private sealed record DomainResponse(string RegistrationId, string Status, string? ZaakUrl);
 }
 

services/bff/Bff.Api/Program.cs

@@ -1,4 +1,6 @@
 using System.Security.Claims;
+using System.Text.Json;
+using System.Text.Json.Serialization;
 using Bff.Api;
 using Microsoft.AspNetCore.Authentication.JwtBearer;
 
@@ -6,6 +8,10 @@ var builder = WebApplication.CreateBuilder(args);
 
 var keycloakAuthority = builder.Configuration["Keycloak:Authority"]
     ?? throw new InvalidOperationException("Missing configuration 'Keycloak:Authority'");
+// Behandelaars authenticate against a *different* Keycloak realm (medewerker) than citizens (digid),
+// so the BFF validates a second issuer for the behandel endpoints (ADR-0013).
+var medewerkerAuthority = builder.Configuration["Keycloak:MedewerkerAuthority"]
+    ?? throw new InvalidOperationException("Missing configuration 'Keycloak:MedewerkerAuthority'");
 var domainBaseUrl = builder.Configuration["Downstream:Domain:BaseUrl"]
     ?? throw new InvalidOperationException("Missing configuration 'Downstream:Domain:BaseUrl'");
 var projectionBaseUrl = builder.Configuration["Downstream:Projection:BaseUrl"]
@@ -19,8 +25,28 @@ builder.Services.AddAuthentication(JwtBearerDefaults.AuthenticationScheme)
         options.Authority = keycloakAuthority;
         options.RequireHttpsMetadata = false;
         options.TokenValidationParameters.ValidateAudience = false;
+    })
+    // The medewerker realm — behandel endpoints only. On validation we lift Keycloak's realm roles
+    // (the nested realm_access.roles claim) into role claims so authorization policies can require them.
+    .AddJwtBearer(BehandelAuth.Scheme, options =>
+    {
+        options.Authority = medewerkerAuthority;
+        options.RequireHttpsMetadata = false;
+        options.TokenValidationParameters.ValidateAudience = false;
+        options.Events = new JwtBearerEvents
+        {
+            OnTokenValidated = context =>
+            {
+                BehandelAuth.AddRealmRoles(context.Principal);
+                return Task.CompletedTask;
+            },
+        };
     });
-builder.Services.AddAuthorization();
+builder.Services.AddAuthorization(options =>
+    options.AddPolicy(BehandelAuth.Policy, policy => policy
+        .AddAuthenticationSchemes(BehandelAuth.Scheme)
+        .RequireAuthenticatedUser()
+        .RequireRole(BehandelAuth.BehandelaarRole)));
 
 // The BFF is the portals' only backend; it fans out to the domain and projection (§8.3).
 builder.Services.AddHttpClient<IDomainClient, DomainClient>(c => c.BaseAddress = new Uri(domainBaseUrl));
@@ -68,7 +94,53 @@ app.MapGet("/openbaar/register", async (string? q, IProjectionClient projection,
 })
     .Produces<IReadOnlyList<OpenbaarEntry>>(StatusCodes.Status200OK);
 
+// Behandelaar's werkbak: registrations awaiting beoordeling. Reached only with a medewerker-realm
+// token carrying the behandelaar role; the BFF proxies the domain's werkbak (staff view, ADR-0013).
+app.MapGet("/behandel/werkbak", async (IDomainClient domain, CancellationToken ct) =>
+    Results.Ok(await domain.GetWerkbakAsync(ct)))
+    .RequireAuthorization(BehandelAuth.Policy)
+    .Produces<IReadOnlyList<WerkbakItem>>(StatusCodes.Status200OK)
+    .Produces(StatusCodes.Status401Unauthorized)
+    .Produces(StatusCodes.Status403Forbidden);
+
 app.Run();
 
+// Behandel (medewerker-realm) authentication + authorization wiring (ADR-0013).
+internal static class BehandelAuth
+{
+    public const string Scheme = "medewerker";
+    public const string Policy = "behandelaar";
+    public const string BehandelaarRole = "behandelaar";
+
+    /// <summary>Lift Keycloak's realm roles (the nested <c>realm_access.roles</c> claim) onto the
+    /// principal as role claims, so <c>RequireRole</c> can authorize on them.</summary>
+    public static void AddRealmRoles(ClaimsPrincipal? principal)
+    {
+        if (principal?.Identity is not ClaimsIdentity identity)
+            return;
+
+        var realmAccess = principal.FindFirst("realm_access")?.Value;
+        if (string.IsNullOrWhiteSpace(realmAccess))
+            return;
+
+        // A malformed realm_access claim must not fail authentication (a throw here becomes a 401);
+        // it simply yields no roles, so the authorization policy answers 403.
+        string[] roles;
+        try
+        {
+            roles = JsonSerializer.Deserialize<RealmAccess>(realmAccess)?.Roles ?? [];
+        }
+        catch (JsonException)
+        {
+            return;
+        }
+
+        foreach (var role in roles)
+            identity.AddClaim(new Claim(identity.RoleClaimType, role));
+    }
+
+    private sealed record RealmAccess([property: JsonPropertyName("roles")] string[] Roles);
+}
+
 // Exposed so the test host (WebApplicationFactory<Program>) can boot the app.
 public partial class Program;

services/bff/Bff.Api/appsettings.json

@@ -7,7 +7,8 @@
   },
   "AllowedHosts": "*",
   "Keycloak": {
-    "Authority": "http://localhost:8180/realms/digid"
+    "Authority": "http://localhost:8180/realms/digid",
+    "MedewerkerAuthority": "http://localhost:8180/realms/medewerker"
   },
   "Downstream": {
     "Domain": { "BaseUrl": "http://localhost:8130/" },