fix: lowercase urls to prevent bypass

[seaerchin]

Problem

go has a blacklist for link shorteners, but this check is a case agnostic check, which means that Bit.ly for example, passes the chekc but bit.ly does not

Solution

1. make teh check case insensitive by converting the url to lowercase in the check. opted against converting the url to lowercase outright incase case matters for the actual url

[Copilot]

Pull request overview

This PR fixes a security vulnerability in the URL blacklist validation where case variations in domain names could bypass blacklist checks. The fix ensures that URLs like https://Bit.ly/abc are properly caught alongside https://bit.ly/abc.

Key Changes:

• Modified isBlacklisted() to perform case-insensitive matching by converting URLs to lowercase before checking against the blacklist

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[dcshzj]

Approving first, also should add tests for this.