refactor: enhance security and performance in configuration and API routes

- Update Content Security Policy (CSP) in next.config.ts to avoid `unsafe-eval` in production, improving security against XSS attacks.
- Refactor API routes to enforce admin authentication and session validation, ensuring secure access to sensitive endpoints.
- Optimize analytics data retrieval by using database aggregation instead of loading all records into memory, improving performance and reducing memory usage.
- Implement session token creation and verification for better session management and security across the application.
- Enhance error handling and input validation in various API routes to ensure robustness and prevent potential issues.

app/api/analytics/performance/route.ts

@@ -4,14 +4,11 @@ import { requireSessionAuth } from '@/lib/auth';
 
 export async function GET(request: NextRequest) {
   try {
-    // Check admin authentication - for admin dashboard requests, we trust the session
+    // Admin-only endpoint
     const isAdminRequest = request.headers.get('x-admin-request') === 'true';
-    if (!isAdminRequest) {
-      const authError = requireSessionAuth(request);
-      if (authError) {
-        return authError;
-      }
-    }
+    if (!isAdminRequest) return NextResponse.json({ error: 'Admin access required' }, { status: 403 });
+    const authError = requireSessionAuth(request);
+    if (authError) return authError;
 
     // Get performance data from database
     const pageViews = await prisma.pageView.findMany({