feat: add linkedin and facebook feature flags, clean up dead code

- Register 'linkedin' FeatureGroup with routers.linkedin and
  api.linkedin_image_generation routers
- Register 'facebook' FeatureGroup with
  api.facebook_writer.routers:facebook_router
- Add 'linkedin' and 'facebook' profiles to PROFILE_GROUP_MAP
- Remove dead imports of linkedin_router, linkedin_image_router,
  and facebook_router from app.py (router manager handles via
  CORE_ROUTER_REGISTRY)
- Add LINKEDIN and FACEBOOK keys to frontend FEATURE_KEYS
- Add route priorities for /linkedin-writer and /facebook-writer
- Change route gates from feature='social' to feature='linkedin'
  and feature='facebook' respectively

frontend/src/App.tsx

@@ -195,8 +195,8 @@ const App: React.FC = () => {
                     <Route path="/seo-dashboard" element={<ProtectedRoute><FeatureRoute feature="seo"><SEODashboard /></FeatureRoute></ProtectedRoute>} />
                     <Route path="/backlink-outreach" element={<ProtectedRoute><FeatureRoute feature="backlinking"><BacklinkOutreachDashboard /></FeatureRoute></ProtectedRoute>} />
                     <Route path="/content-planning" element={<ProtectedRoute><FeatureRoute feature="content-planning"><ContentPlanningDashboard /></FeatureRoute></ProtectedRoute>} />
-                    <Route path="/facebook-writer" element={<ProtectedRoute><FeatureRoute feature="social"><FacebookWriter /></FeatureRoute></ProtectedRoute>} />
-                    <Route path="/linkedin-writer" element={<ProtectedRoute><FeatureRoute feature="social"><LinkedInWriter /></FeatureRoute></ProtectedRoute>} />
+                    <Route path="/facebook-writer" element={<ProtectedRoute><FeatureRoute feature="facebook"><FacebookWriter /></FeatureRoute></ProtectedRoute>} />
+                    <Route path="/linkedin-writer" element={<ProtectedRoute><FeatureRoute feature="linkedin"><LinkedInWriter /></FeatureRoute></ProtectedRoute>} />
                     <Route path="/blog-writer" element={<ProtectedRoute><FeatureRoute feature="blog_writer"><BlogWriter /></FeatureRoute></ProtectedRoute>} />
                     <Route path="/story-writer" element={<ProtectedRoute><FeatureRoute feature="story"><StoryWriter /></FeatureRoute></ProtectedRoute>} />
                     <Route path="/story-projects" element={<ProtectedRoute><FeatureRoute feature="story"><StoryProjectList /></FeatureRoute></ProtectedRoute>} />

frontend/src/utils/demoMode.ts

@@ -16,6 +16,8 @@ export const FEATURE_KEYS = {
   SEO: 'seo',
   CONTENT_PLANNING: 'content-planning',
   SOCIAL: 'social',
+  LINKEDIN: 'linkedin',
+  FACEBOOK: 'facebook',
   BLOG_WRITER: 'blog_writer',
   STORY: 'story',
   YOUTUBE: 'youtube',
@@ -126,6 +128,8 @@ const FEATURE_ROUTE_PRIORITY: [string, string][] = [
   ['podcast', '/podcast-maker'],
   ['blog_writer', '/blog-writer'],
   ['backlinking', '/backlink-outreach'],
+  ['linkedin', '/linkedin-writer'],
+  ['facebook', '/facebook-writer'],
   ['story', '/story-writer'],
   ['image', '/image-studio'],
   ['video', '/video-studio'],