🛡️ Sentinel: [CRITICAL] Fix PRAGMA SQL injection vulnerability

[mapleleaflatte03]

🚨 Severity: CRITICAL
💡 Vulnerability: Unvalidated environment variable (MERIDIAN_OBSERVABILITY_SQLITE_JOURNAL_MODE) is used to directly format a SQLite PRAGMA statement, leading to a SQL injection vulnerability.
🎯 Impact: A malicious user or compromised environment could inject arbitrary SQLite commands, leading to data deletion or database corruption.
🔧 Fix: Validated the environment variable against a strict allowlist of known-good SQLite journal modes ('DELETE', 'TRUNCATE', 'PERSIST', 'MEMORY', 'WAL', 'OFF', 'DEFAULT') before executing the PRAGMA statement, and fell back to the safe default ('WAL') if validation fails.
✅ Verification: Ran python3 -m unittest discover on the observability platform tests. Confirmed that no new errors were introduced by the stricter journal mode parsing.

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PR created automatically by Jules for task 9905149795299783722 started by @mapleleaflatte03

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