feat(projects): gate Project Overview behind a projects feature flag + cockpit email actions

- Migration 120 seeds the projects flag (default OFF), idempotent.
- Backend feature-flags whitelist + DEFAULT_FLAGS + clients derivation.
- adminProjects routes 403 PROJECTS_DISABLED when the flag is off.
- projectService email actions (resend/cancel/retry/send-now) + routes.
- Frontend flag type, DEFAULT_FLAGS, Features tab card (en+de).

frontend/src/contexts/FeatureFlagsContext.tsx

@@ -45,6 +45,10 @@ export const DEFAULT_FLAGS: FeatureFlags = {
   // Settings → Features once they've reviewed the seeded block
   // library with their lawyer.
   contracts: false,
+  // Projects (migration 120). Admin-only grouping layer above events +
+  // the Project Overview cockpit. Off by default — admin opts in under
+  // Settings → Features once they want the CRM → Overview area.
+  projects: false,
 };
 
 export const FEATURE_FLAGS_QUERY_KEY = ['feature-flags'] as const;