feat(stamdata): admin stamdata maintenance editor (beheer)

Realizes ADR-0004's "future low-code editor that commits a PR": an
admin-only stamdata maintenance editor built on the stamdata-as-code
foundation.

Backend: `professions` moves from a hardcoded C# dictionary to an embedded
`professions.json` data-file (typed as `ProfessionMapping`) with valid-time
(geldigVan/geldigTot, half-open). A generic, reflection-driven
StamdataCatalog/StamdataTable/StamdataFile describes every table so one
endpoint pair + one grid editor serve all of them; add a table in one line.
Two read-only, admin-gated endpoints (GET /stamdata, GET /stamdata/{table}
?peildatum=) — no runtime write path. Generic build gate
`Every_catalog_table_is_valid` (keys non-blank, no overlapping validity,
well-formed windows).

Frontend: new `beheer` context (route beheer/stamdata, capabilityGuard
'stamdata:edit'). A schema-driven grid editor edits rows locally; download()
emits {table}.json for the admin to commit as a reviewed PR (no mutation
command — the CI build + StamdataValidationTests stay the authority).

Full gate GREEN both sides; gen:api leaves no drift; new stamdata story
passes axe. See WP-29 + ADR-0004.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/BigRegister.Api/Domain/Authorization/Authz.cs

@@ -43,7 +43,7 @@ public static class Authz
   public static IReadOnlyList<string> RoleCapabilities(Principal principal) => principal.Role switch
   {
     PrincipalRole.Approver => new[] { "brief:approve", "brief:reject", "brief:send" },
-    PrincipalRole.Admin => new[] { "orgtemplate:edit" },
+    PrincipalRole.Admin => new[] { "orgtemplate:edit", "stamdata:edit" },
     _ => Array.Empty<string>(),
   };
 
@@ -64,6 +64,11 @@ public static class Authz
   /// have no per-resource state to weigh, so role IS the whole decision here.
   public static bool CanManageOrgTemplates(Principal principal) => principal.Role == PrincipalRole.Admin;
 
+  /// Stamdata maintenance (ADR-0004): admin-only, resource-independent — same shape as
+  /// org-template management (role IS the decision). Gates the read-only /stamdata endpoints
+  /// the maintenance editor consumes; the actual edit lands as a reviewed PR, not a write here.
+  public static bool CanEditStamdata(Principal principal) => principal.Role == PrincipalRole.Admin;
+
   /// Field-level PII (PRD-0002 §5c, phase P2): the case screen's BIG-nummer ships
   /// masked by default; only the behandelaar (Drafter) composing the case — the actor
   /// whose behandel-scherm shows the field — may reveal it. Role-based in the POC; a