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

[ightevenmckane187]

🛡️ Sentinel Security Patch:

• Vulnerability: Non-constant-time string comparison (!=) in verify_api_key (auth.py) exposed the API key authentication gate to side-channel timing analysis attacks.
• Fix: Substituted string equality operator with hmac.compare_digest(provided_key, CYPHER_API_KEY) to enforce constant-time API key comparisons.
• Verification: Added test_api_create_share_link_timing_safety in tests/test_share_system.py asserting rejection of invalid keys of various lengths and formats. Updated .gitignore to prevent database test artifacts from tracking. All 24 Python unit tests and 199 Jest tests pass.

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PR created automatically by Jules for task 2947057660451506288 started by @ightevenmckane187

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