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/src/BigRegister.Api/Data/Migrations/AppDbContextModelSnapshot.cs

@@ -184,6 +184,19 @@ namespace BigRegister.Api.Data.Migrations
                     b.ToTable("Briefs");
                 });
 
+            modelBuilder.Entity("BigRegister.Api.Data.FeatureFlagEntity", b =>
+                {
+                    b.Property<string>("Key")
+                        .HasColumnType("TEXT");
+
+                    b.Property<bool>("Enabled")
+                        .HasColumnType("INTEGER");
+
+                    b.HasKey("Key");
+
+                    b.ToTable("FeatureFlags");
+                });
+
             modelBuilder.Entity("BigRegister.Api.Data.OrgTemplateEntity", b =>
                 {
                     b.Property<string>("SubOrgId")