feat: Make backend port configurable via PORT environment variable (defaults to 3001)

This allows changing the port without editing code:

src/backend/index.js

@@ -679,12 +679,13 @@ const app = new Elysia()
     } catch {
       return new Response('Frontend not built. Run `bun run build`', { status: 503 });
     }
-  })
-
-  // Start server
-  .listen(3001);
 
 logger.info(`Backend server running`, { port: app.server?.port, url: `http://localhost:${app.server?.port}` });
 logger.info(`API endpoints available`, { url: `http://localhost:${app.server?.port}/api` });
 
+// Start server - uses PORT environment variable if set, otherwise defaults to 3001
+const PORT = process.env.PORT || 3001;
+
+.listen(PORT);
+
 export default app;