🛡️ Sentinel: [HIGH] Fix SQL injection in SQLite PRAGMA journal_mode

[mapleleaflatte03]

🚨 Severity: HIGH
💡 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 query.
🎯 Impact: An attacker with the ability to control environment variables could execute arbitrary SQLite commands (e.g., WAL; DROP TABLE...) leading to data loss or corruption.
🔧 Fix: Added strict allowlist validation for the environment variable against valid SQLite journal modes (DELETE, TRUNCATE, PERSIST, MEMORY, WAL, OFF, DEFAULT, ""). If an invalid value is provided, it defaults safely to WAL.
✅ Verification: Verified by running the local test suites to ensure normal application behavior is not disrupted, and confirmed visually via git diff.

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PR created automatically by Jules for task 1124750764782372377 started by @mapleleaflatte03

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