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

[mapleleaflatte03]

🚨 Severity: CRITICAL
💡 Vulnerability: SQL injection was possible through an unvalidated environment variable MERIDIAN_OBSERVABILITY_SQLITE_JOURNAL_MODE when dynamically generating the PRAGMA journal_mode=... statement in intelligence/company/meridian_platform/observability_store.py. Because the sqlite3 module does not support parameterized query execution (?) for PRAGMA statements, the environment variable was directly interpolated into the SQL string, creating an injection vector.
🎯 Impact: A malicious actor who gained the ability to modify environment variables on the host could execute arbitrary SQL statements against the observability stream database by injecting statements like OFF; DROP TABLE audit_events;-- as the journal mode, resulting in complete database compromise.
🔧 Fix: Implemented an explicit validation allowlist for MERIDIAN_OBSERVABILITY_SQLITE_JOURNAL_MODE. The value is now strictly checked against a known-safe set of valid SQLite journal modes (DELETE, TRUNCATE, PERSIST, MEMORY, WAL, OFF) and allowed fallback values ('', DEFAULT). If the supplied value is not in the allowlist, the code safely defaults to WAL.
✅ Verification: Ran the Meridian Platform unit test suite (python3 -m unittest discover -s intelligence/company/meridian_platform -p "test_*.py") and confirmed no new failures were introduced. Code review was passed.

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PR created automatically by Jules for task 3700974545461866479 started by @mapleleaflatte03

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