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

[mapleleaflatte03]

🚨 Severity: CRITICAL
💡 Vulnerability: SQL injection vulnerability in observability_store.py where an unvalidated environment variable was interpolated directly into a PRAGMA journal_mode SQL query.
🎯 Impact: An attacker who can control environment variables could inject arbitrary SQL code, potentially leading to unauthorized data access or modification.
🔧 Fix: Implemented an explicit allowlist of valid SQLite journal modes to strictly validate the environment variable before executing the PRAGMA query.
✅ Verification: Verified that only valid modes are accepted and tested the fallback mechanism when invalid inputs are provided.

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PR created automatically by Jules for task 9290338889244090994 started by @mapleleaflatte03

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