feat(brief): WP-18 — ABAC capability spine (PRD-0002 phase P1)

Replace the FE-computed authorization anti-pattern in BriefStore.editable
(derived from the unverified X-Role header) with server-computed decision
flags, mirroring the existing HerregistratieDecisionsDto pattern:

- Backend: Authz.cs is the single authorization helper — the SAME check
  (Authz.CanActOn) both gates BriefStore.Review's mutations and computes
  the BriefDecisionsDto flags shipped on every brief response, so emit
  and enforce can never drift. New GET /me returns coarse, role-derived
  capabilities (PRD-0002 SS6).
- Every brief endpoint (including send, previously ungated on HttpContext)
  now returns a fresh BriefViewDto so decisions never go stale after a
  mutation.
- FE: brief.store.ts reads canEdit/canApprove/canReject/canSend off the
  loaded decisions instead of computing them from currentRole(); the
  brief.machine carries decisions through every status transition.
- New shared/domain/capability.ts + shared/application/access.store.ts +
  shared/infrastructure/me.adapter.ts: the general capability-spine
  infrastructure (AccessStore.can(), capabilityGuard) for future routes.

Deviates from the original WP-18 draft by NOT renaming auth/domain's
Session to a Principal union — ADR-0002 explicitly defers that refactor
until a second actor exists, and the brief workflow's drafter/approver
identity turned out to be a separate axis from the SSP login session
entirely. See docs/backlog/WP-18-abac-capability-spine.md for the full
as-built record.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/src/BigRegister.Api/Program.cs

@@ -2,6 +2,7 @@ using System.Text.Json;
 using System.Text.Json.Serialization;
 using BigRegister.Api.Contracts;
 using BigRegister.Api.Data;
+using BigRegister.Domain.Authorization;
 using BigRegister.Domain.Diplomas;
 using BigRegister.Domain.Documents;
 using BigRegister.Domain.Intake;
@@ -239,76 +240,81 @@ api.MapPost("/applications/{id}/submit", (string id, SubmitApplicationRequest re
 .ProducesProblem(StatusCodes.Status409Conflict)
 .Produces(StatusCodes.Status404NotFound);
 
-// --- Brief (letter composition). One demo brief per owner; the server owns the
-// status machine + role rules. Role is a dev-only stand-in via X-Role (mirrors the
-// X-Admin seam and the FE ?role= toggle) — no real identities in this POC. ---
+// PRD-0002 §6: coarse, role-derived capabilities for nav/menu-level checks (NOT
+// tied to a specific brief's live status — see BriefDecisionsDto for that).
+api.MapGet("/me", (HttpContext ctx) => new MeDto(Authz.RoleCapabilities(Authz.ResolvePrincipal(ctx))))
+.Produces<MeDto>();
 
-api.MapGet("/brief", () =>
+// --- Brief (letter composition). One demo brief per owner; the server owns the
+// status machine + authorization (Authz, PRD-0002 phase P1). Principal is a
+// dev-only stand-in via X-Role (mirrors the X-Admin seam and the FE ?role=
+// toggle) — no real identities in this POC. ---
+
+api.MapGet("/brief", (HttpContext ctx) =>
 {
   var e = BriefStore.GetOrCreate(DocumentStore.DemoOwner);
-  return new BriefViewDto(e.ToDto(), BriefSeed.PassagesFor(e.Beroep));
+  return ToView(ctx, e);
 })
 .Produces<BriefViewDto>();
 
 api.MapPut("/brief", (SaveBriefRequest req, HttpContext ctx) =>
 {
-  var (_, isDrafter) = BriefRole(ctx);
-  return BriefResult(BriefStore.Save(DocumentStore.DemoOwner, req.Sections, isDrafter), "Alleen de opsteller mag de brief bewerken.");
+  var isDrafter = Authz.ResolvePrincipal(ctx).Role == PrincipalRole.Drafter;
+  return BriefResult(ctx, BriefStore.Save(DocumentStore.DemoOwner, req.Sections, isDrafter), "Alleen de opsteller mag de brief bewerken.");
 })
-.Produces<BriefDto>()
+.Produces<BriefViewDto>()
 .ProducesProblem(StatusCodes.Status403Forbidden)
 .ProducesProblem(StatusCodes.Status409Conflict);
 
 api.MapPost("/brief/submit", (HttpContext ctx) =>
 {
-  var (_, isDrafter) = BriefRole(ctx);
+  var isDrafter = Authz.ResolvePrincipal(ctx).Role == PrincipalRole.Drafter;
   var r = BriefStore.Submit(DocumentStore.DemoOwner, isDrafter, Now());
   LogBrief("submit", r);
-  return BriefResult(r, "Alleen de opsteller mag indienen.");
+  return BriefResult(ctx, r, "Alleen de opsteller mag indienen.");
 })
 .WithName("briefSubmit") // distinct name so the generated client method isn't `submit2`
-.Produces<BriefDto>()
+.Produces<BriefViewDto>()
 .ProducesProblem(StatusCodes.Status403Forbidden)
 .ProducesProblem(StatusCodes.Status409Conflict);
 
 api.MapPost("/brief/approve", (HttpContext ctx) =>
 {
-  var (acting, _) = BriefRole(ctx);
-  var r = BriefStore.Approve(DocumentStore.DemoOwner, acting, Now());
+  var r = BriefStore.Approve(DocumentStore.DemoOwner, Authz.ResolvePrincipal(ctx), Now());
   LogBrief("approve", r);
-  return BriefResult(r, "De beoordelaar mag niet de opsteller zijn.");
+  return BriefResult(ctx, r, "De beoordelaar mag niet de opsteller zijn.");
 })
-.Produces<BriefDto>()
+.Produces<BriefViewDto>()
 .ProducesProblem(StatusCodes.Status403Forbidden)
 .ProducesProblem(StatusCodes.Status409Conflict);
 
 api.MapPost("/brief/reject", (RejectBriefRequest req, HttpContext ctx) =>
 {
-  var (acting, _) = BriefRole(ctx);
-  var r = BriefStore.Reject(DocumentStore.DemoOwner, acting, req.Comments, Now());
+  var r = BriefStore.Reject(DocumentStore.DemoOwner, Authz.ResolvePrincipal(ctx), req.Comments, Now());
   LogBrief("reject", r);
-  return BriefResult(r, "De beoordelaar mag niet de opsteller zijn.");
+  return BriefResult(ctx, r, "De beoordelaar mag niet de opsteller zijn.");
 })
-.Produces<BriefDto>()
+.Produces<BriefViewDto>()
 .ProducesProblem(StatusCodes.Status403Forbidden)
 .ProducesProblem(StatusCodes.Status409Conflict);
 
-api.MapPost("/brief/send", () =>
+api.MapPost("/brief/send", (HttpContext ctx) =>
 {
   // Send-time placeholder linting is FE-authoritative in this slice (no C# parity
-  // port); the backend only guards the approved→sent transition.
+  // port); the backend only guards the approved→sent transition (not role-gated
+  // today — see Authz.CanActOn(Send, …), a mechanical dispatch step).
   var r = BriefStore.Send(DocumentStore.DemoOwner, Now());
   LogBrief("send", r);
-  return BriefResult(r, "Versturen kan niet in deze status.");
+  return BriefResult(ctx, r, "Versturen kan niet in deze status.");
 })
-.Produces<BriefDto>()
+.Produces<BriefViewDto>()
 .ProducesProblem(StatusCodes.Status409Conflict);
 
-api.MapPost("/brief/reset", () =>
+api.MapPost("/brief/reset", (HttpContext ctx) =>
 {
   // Demo "start over": recreate a fresh draft. No guards — showcase affordance only.
   var e = BriefStore.ResetAndCreate(DocumentStore.DemoOwner);
-  return new BriefViewDto(e.ToDto(), BriefSeed.PassagesFor(e.Beroep));
+  return ToView(ctx, e);
 })
 .WithName("briefReset")
 .Produces<BriefViewDto>();
@@ -319,17 +325,16 @@ static bool IsAdmin(HttpContext ctx) => ctx.Request.Headers["X-Admin"] == "true"
 
 static string Now() => DateTimeOffset.UtcNow.ToString("o");
 
-// Dev-only role stand-in: X-Role: approver acts as the approver identity, anything
-// else (default) acts as the drafter.
-static (string acting, bool isDrafter) BriefRole(HttpContext ctx)
-{
-  var isDrafter = ctx.Request.Headers["X-Role"].ToString() != "approver";
-  return (isDrafter ? BriefStore.DrafterId : BriefStore.ApproverId, isDrafter);
-}
+BriefViewDto ToView(HttpContext ctx, BriefEntity e) => new(
+    e.ToDto(),
+    BriefSeed.PassagesFor(e.Beroep),
+    Authz.Decisions(Authz.ResolvePrincipal(ctx), e.Status.Tag, e.DrafterId));
 
-IResult BriefResult((BriefStore.Outcome outcome, BriefEntity? entity) r, string forbiddenDetail) => r.outcome switch
+// Emit (decision flags, via ToView) and enforce (Forbidden/Conflict below) both run
+// through Authz — see BriefStore.Review and Authz.CanActOn — so they cannot drift.
+IResult BriefResult(HttpContext ctx, (BriefStore.Outcome outcome, BriefEntity? entity) r, string forbiddenDetail) => r.outcome switch
 {
-  BriefStore.Outcome.Ok => Results.Ok(r.entity!.ToDto()),
+  BriefStore.Outcome.Ok => Results.Ok(ToView(ctx, r.entity!)),
   BriefStore.Outcome.Forbidden => Results.Problem(detail: forbiddenDetail, statusCode: StatusCodes.Status403Forbidden),
   _ => Results.Problem(detail: "Ongeldige overgang voor de huidige status van de brief.", statusCode: StatusCodes.Status409Conflict),
 };