feat(security): implement Next.js Edge Middleware for admin routes

[martinshub-tech]

Closes #1087

Description: This PR implements Next.js Edge Middleware to securely protect administrative routes, directly fulfilling the required technical specifications and edge deployment considerations.

Key Features & Implementation Details:

JWT & Session Validation: Validates JWT signatures and session nonces (x-nonce) directly on the Next.js Edge runtime using standard Web Crypto API (crypto.subtle.verify), with zero origin server overhead.
IP Allowlisting & Geo-fencing: Enforces region restrictions (ALLOWED_ADMIN_COUNTRIES) and IP whitelists (ALLOWED_ADMIN_IPS) for all paths matching /admin*.
Bot Detection & Rate Limiting: Introduces early request termination for automated malicious agents, paired with an efficient in-memory token-bucket mechanism deployed at the edge layer.
Secure Redirect Handling: Preserves return paths on unauthenticated requests and automatically validates the returnTo URLs against open-redirect vulnerabilities.
Testing Notes:

JWT_SECRET must be set in your .env to validate admin tokens locally.
Adjust ALLOWED_ADMIN_IPS (comma-separated list of IPs) to simulate the IP allowlist. By default, localhost traffic is permitted.
Run pnpm dev and visit /admin to verify that unauthorized requests successfully redirect to /login?returnTo=/admin

[drips-wave]

@martinshub-tech Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits

[vercel]

@martinshub-tech is attempting to deploy a commit to the Ayomide Adeniran's projects Team on Vercel.

A member of the Team first needs to authorize it.

[adeniran19-maker]

@martinshub-tech PR under review — I will get back to you if I find any wrong implementations.