🛡️ Sentinel: [HIGH] Fix SQL injection vector in PRAGMA statement

[mapleleaflatte03]

🚨 Severity: HIGH
💡 Vulnerability: SQL injection vector via dynamically generated PRAGMA query in observability_store.py. The MERIDIAN_OBSERVABILITY_SQLITE_JOURNAL_MODE environment variable was interpolated directly into conn.execute(f'PRAGMA journal_mode={configured_journal_mode}') without strict allowlisting.
🎯 Impact: A malicious actor who can control the environment variable could pass malformed strings or additional statements (though execute() limits to one statement, it still allows invalid PRAGMA values or potential engine crashes) due to the lack of parameterized queries for PRAGMAs.
🔧 Fix: Added a strict validation check using an explicit allowlist of acceptable SQLite journal modes ('DELETE', 'TRUNCATE', 'PERSIST', 'MEMORY', 'WAL', '', 'DEFAULT', 'OFF').
✅ Verification: Ran unit tests to ensure no regressions were introduced. Evaluated manually by attempting to inject invalid strings, which correctly fallback to 'WAL'.

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PR created automatically by Jules for task 1407202302671966321 started by @mapleleaflatte03

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