feat(workflows): add workflows feature flag + Features-tab toggle

New opt-in 'workflows' master flag (default off) across the backend
KNOWN_FLAGS/DEFAULT_FLAGS and the frontend FeatureKey union, context
defaults, and a new Automation section card in the Features tab. Gates
the upcoming Workflows admin area and the engine runtime. en/de i18n
added (DE native).

frontend/src/contexts/FeatureFlagsContext.tsx

@@ -64,6 +64,9 @@ export const DEFAULT_FLAGS: FeatureFlags = {
   whatsapp: false,
   // Live Slideshow ("Diashow") — opt-in; gates all slideshow admin UI.
   slideshow: false,
+  // Workflow / automation engine — opt-in; gates the Workflows admin area
+  // and the engine runtime (triggers/actions/gates).
+  workflows: false,
 };
 
 export const FEATURE_FLAGS_QUERY_KEY = ['feature-flags'] as const;