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>

docs/project/backlog/README.md

@@ -91,6 +91,7 @@ for its existing violations, so every WP ends green.
 | [WP-44](WP-44-context-generator.md)            | Runnable generator: `gen:context`                                                  | 8 · platform/DX/showcase    | todo   |
 | [WP-45](WP-45-create-ssp-generator.md)         | `create-ssp` bootstrap generator (mechanise new-ssp)                               | 8 · platform/DX/showcase    | todo   |
 | [WP-46](WP-46-vitest-coverage.md)              | Vitest coverage (report + report-only thresholds)                                  | 8 · platform/DX/showcase    | done   |
+| [WP-47](WP-47-feature-flags.md)                | Runtime feature flags (catalog-in-code, admin toggle, FE+backend)                  | 8 · platform/DX/showcase    | done   |
 
 Sequencing dependencies (stated in the WPs too): 01 before 10–15 (axe covers story churn);
 03/04 before 05–09 (boundaries stop new violations during refactors); 06 before 07 (typed