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

[mapleleaflatte03]

🚨 Severity: CRITICAL
💡 Vulnerability: The SQLite observability store directly interpolates the MERIDIAN_OBSERVABILITY_SQLITE_JOURNAL_MODE environment variable into a PRAGMA statement using string concatenation. Since PRAGMA queries in SQLite do not support parameterized variables (?), this exposes the system to SQL injection attacks if a malicious configuration is provided.
🎯 Impact: A maliciously configured environment variable could execute arbitrary SQL commands during the database connection phase, leading to potential data destruction or unauthorized data manipulation in the observability logs.
🔧 Fix: Added a strict allowlist validation for configured_journal_mode. If the environment variable provides a mode that isn't one of the known safe operations (e.g., WAL, DELETE, OFF), it safely falls back to the default WAL.
✅ Verification: Verified the change by testing an unsafe environment injection and ensuring it defaults to WAL, and confirmed via local tests that regressions weren't introduced.

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PR created automatically by Jules for task 1854527127887819927 started by @mapleleaflatte03

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