Allow all users to generate AI text for posts

Change API endpoint `/api/ai/generate` from `requireAdmin` to `requireAuth` middleware and update OpenRouterService to use shared admin configuration instead of per-user config.

Replit-Commit-Author: Agent
Replit-Commit-Session-Id: ae4037a0-2a6f-4530-9bac-79b543286bda
Replit-Commit-Checkpoint-Type: intermediate_checkpoint
Replit-Commit-Screenshot-Url: https://storage.googleapis.com/screenshot-production-us-central1/397bca8c-984f-43ff-841a-10897aeb8140/ae4037a0-2a6f-4530-9bac-79b543286bda/tWDOGLP

server/routes.ts

@@ -435,7 +435,7 @@ export async function registerRoutes(app: Express): Promise<Server> {
   });
 
   // AI text generation
-  app.post("/api/ai/generate", requireAdmin, async (req, res) => {
+  app.post("/api/ai/generate", requireAuth, async (req, res) => {
     try {
       const user = req.user as User;
       const userId = user.id;