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/features/settings/tabs/FeaturesTab.tsx

@@ -16,6 +16,7 @@ import {
   Briefcase,
   Wrench,
   Calculator,
+  FolderKanban,
 } from 'lucide-react';
 import { useTranslation } from 'react-i18next';
 import { Button, Card } from '../../../components/common';
@@ -291,6 +292,20 @@ export const FeaturesTab: React.FC = () => {
             enabled={staged.hoursLogging}
             onToggle={(next) => setFlag('hoursLogging', next)}
           />
+
+          <FeatureCard
+            icon={FolderKanban}
+            title={t('settings.features.projects.title', 'Projects')}
+            description={t(
+              'settings.features.projects.description',
+              'Admin-only grouping layer above events. Bundle several events under one project and open a 360° Project Overview cockpit — milestone timeline plus a dated feed of every email (with the actual sent preview + resend/cancel/retry actions), quote, contract, invoice, gallery and logged hour. Adds a "book to project" control when logging hours. Customers never see projects.',
+            )}
+            status="new"
+            statusLabel={statusLabel('new')}
+            sidebarLabel={t('settings.features.projects.sidebar', 'Overview')}
+            enabled={staged.projects}
+            onToggle={(next) => setFlag('projects', next)}
+          />
         </Section>
 
         {/* Insights & Access */}