feat(admin): runtime feature flags (catalog-in-code, admin toggle, FE+backend)

Catalog declared in code (Domain/Features/FeatureFlags.cs, build-validated), on/off state
persisted in SQLite (FeatureFlagStore + migration). GET /flags (drives FE gating) + admin
PUT /admin/flags/{key} (new flags:manage capability + FlagsAdmin gate). Enforced end-to-end:
the `inschrijving-open` flag hides the Inschrijven nav item + dashboard action (FE) AND makes
POST /applications for a registratie 403 when off (backend). FE FeatureFlagStore mirrors
AccessStore (enabled() deny-by-default); admin toggle page at /beheer/functies in ADMIN_LINKS.
+4 backend tests, /me cap-list updated, client regenerated.

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

backend/tests/BigRegister.Tests/OrgTemplateEndpointTests.cs

@@ -201,7 +201,9 @@ public class OrgTemplateEndpointTests(TestWebApplicationFactory factory) : IClas
   {
     var res = await _client.SendAsync(Req(HttpMethod.Get, "/api/v1/me", role: "admin"));
     var me = await res.Content.ReadFromJsonAsync<MeDto>();
-    Assert.Equal(new[] { "orgtemplate:edit", "stamdata:edit", "cases:manage" }, me!.Capabilities);
+    Assert.Equal(
+        new[] { "orgtemplate:edit", "stamdata:edit", "cases:manage", "flags:manage" },
+        me!.Capabilities);
   }
 
   [Fact]