🛡️ Sentinel: [CRITICAL] Fix SQL injection in SQLite PRAGMA configuration

[mapleleaflatte03]

🚨 Severity: CRITICAL
💡 Vulnerability: SQL injection vulnerability via the MERIDIAN_OBSERVABILITY_SQLITE_JOURNAL_MODE environment variable in observability_store.py. PRAGMA statements in sqlite3 do not support parameterized queries, so direct concatenation of environment variables into the PRAGMA query string allows SQL injection.
🎯 Impact: An attacker who can control or influence this environment variable could inject arbitrary SQL commands or manipulate SQLite connection behavior, leading to data corruption or potentially further compromise depending on the execution context.
🔧 Fix: Implemented strict validation of the dynamically generated PRAGMA input against an explicit allowlist of valid SQLite journal modes ('DELETE', 'TRUNCATE', 'PERSIST', 'MEMORY', 'WAL', 'OFF', 'DEFAULT', ''). Invalid values safely fall back to the default 'WAL' mode.
✅ Verification: Tested the new logic locally by attempting to inject malicious PRAGMA payloads and verifying the fallback logic successfully mitigates the injection while still allowing valid modes to function correctly. Also successfully ran the core test suite (PYTHONPATH=intelligence/company/meridian_platform python3 -m unittest discover -s intelligence -p "test_*.py") with no new regressions.

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PR created automatically by Jules for task 15337542983911085825 started by @mapleleaflatte03

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