🛡️ Sentinel: Fix XSS protocol obfuscation in announcement link validation

[one-more-refactor]

🚨 Severity: HIGH
💡 Vulnerability: Simple prefix string matching for announcement links allowed obfuscated dangerous protocol URLs (e.g. control characters inserted into javascript: or data: protocols) or protocol-relative external redirects.
🎯 Impact: Potential XSS or unintended navigation if malicious URLs were saved into public site announcement banners.
🔧 Fix: Enhanced URL validation by stripping ASCII control characters and null bytes, parsing with standard URL constructor, ensuring protocol is http: or https:, and disallowing protocol-relative // URLs.
✅ Verification: Added unit tests in src/announcement_url.test.ts, ran bun test, bun run check, and bun run build.

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PR created automatically by Jules for task 6961485221136022041 started by @one-more-refactor

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