🛡️ Sentinel: Fix timing side-channel vulnerability in API key verification

[ightevenmckane187]

🛡️ Sentinel Security Improvement

• Vulnerability: Standard string equality (!=) in verify_api_key in auth.py was vulnerable to timing side-channel attacks where an attacker could measure execution duration differences to deduce valid API key/token characters.
• Fix: Replaced standard inequality check with hmac.compare_digest(provided_key, CYPHER_API_KEY) to ensure constant-time comparison.
• Testing: Added test_api_create_share_link_timing_safety in tests/test_share_system.py asserting proper 401/403 responses across various key formats and lengths. All Python and TypeScript unit test suites pass cleanly.

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PR created automatically by Jules for task 10152525788508241593 started by @ightevenmckane187

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