🛡️ Sentinel: [CRITICAL] Fix SQL injection in metadata generator

[thebearwithabite]

🚨 Severity: CRITICAL
💡 Vulnerability: The save_file_metadata method in metadata_generator.py dynamically constructed SQL INSERT OR REPLACE INTO statements using unvalidated dictionary keys (metadata.keys()) as column names. This exposed the application to SQL injection since column names cannot be parameterized.
🎯 Impact: An attacker controlling the metadata keys could execute arbitrary SQL commands (e.g. drop tables, data exfiltration) or bypass intended access controls.
🔧 Fix: Fetched valid column names directly from the SQLite schema using PRAGMA table_info(file_metadata) to create a strict allowlist. Filtered the incoming dictionary keys against this allowlist before dynamically constructing the SQL query string.
✅ Verification: Ran test_metadata.py to ensure the metadata generator functions correctly with the applied column filtering.

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PR created automatically by Jules for task 5516510970673352591 started by @thebearwithabite

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