🛡️ Sentinel: Fix URL protocol validation to prevent XSS

[one-more-refactor]

🚨 Severity: HIGH
💡 Vulnerability: Simple prefix string matches allowed obfuscated JS protocols (e.g., via ASCII control characters or null bytes) to bypass URL validation.
🎯 Impact: Attackers could inject malicious javascript: or data: URIs in announcement links resulting in stored XSS on the user site.
🔧 Fix: Stripped ASCII control characters and null bytes ('[\x00-\x1F\x7F]') and validated protocol using the standard URL constructor.
✅ Verification: Added unit tests in src/url_validation.test.ts and ran bun test, bun run check, and bun run build.

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PR created automatically by Jules for task 9654341454508009415 started by @one-more-refactor

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