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/contacts/route.ts

@@ -1,10 +1,15 @@
 import { type NextRequest, NextResponse } from "next/server";
 import { PrismaClientKnownRequestError } from '@prisma/client/runtime/library';
-import { checkRateLimit, getRateLimitHeaders } from '@/lib/auth';
+import { checkRateLimit, getRateLimitHeaders, requireSessionAuth } from '@/lib/auth';
 import { prisma } from '@/lib/prisma';
 
 export async function GET(request: NextRequest) {
     try {
+        const isAdminRequest = request.headers.get('x-admin-request') === 'true';
+        if (!isAdminRequest) return NextResponse.json({ error: 'Admin access required' }, { status: 403 });
+        const authError = requireSessionAuth(request);
+        if (authError) return authError;
+
         const { searchParams } = new URL(request.url);
         const filter = searchParams.get('filter') || 'all';
         const limit = parseInt(searchParams.get('limit') || '50');