🛡️ Sentinel: [CRITICAL] Fix SQL injection in PRAGMA statement

[mapleleaflatte03]

🚨 Severity: CRITICAL

💡 Vulnerability: SQL injection vulnerability in observability_store.py where the MERIDIAN_OBSERVABILITY_SQLITE_JOURNAL_MODE environment variable was directly interpolated into a PRAGMA journal_mode={...} SQL statement without validation.

🎯 Impact: Since PRAGMA statements do not support parameterized queries (?), unvalidated dynamic input allows executing arbitrary SQL statements by appending them separated by semicolons. This could result in bypassing security controls or modifying underlying data.

🔧 Fix: Instead of direct string interpolation, the dynamic journal mode value is strictly validated against an explicit allowlist of known-safe SQLite journal modes (and accepted historical bypasses). Unknown values default to a safe value (WAL).

✅ Verification: Ensured via code logic that the environment variable input can only take safely predefined values before executing PRAGMA. Unit tests via PYTHONPATH=intelligence/company/meridian_platform python3 -m unittest discover -s intelligence/company/meridian_platform -p "test_alerting.py" execute and pass without regressions.

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PR created automatically by Jules for task 10615744975384078154 started by @mapleleaflatte03

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