🛡️ Sentinel: [HIGH] Fix ReDoS vulnerability in search scoring

[hadsern]

🚨 Severity: HIGH
💡 Vulnerability: Passing unescaped user input directly into new RegExp() allows Regular Expression Denial of Service (ReDoS) attacks and crashes via unhandled SyntaxError when given malformed input like *.
🎯 Impact: An attacker could cause the search endpoint (/api/ask) to hang or crash, resulting in a Denial of Service.
🔧 Fix: Replaced the regular expression counting logic with a safe, string-based indexOf loop that handles all string input safely without evaluating regex syntax. Added an empty string check to prevent infinite loops.
✅ Verification: Ran pnpm test, pnpm lint, and pnpm build successfully. Evaluated indexOf replacement via local test scripts.

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PR created automatically by Jules for task 13809575176820434846 started by @hadsern

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