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

[mapleleaflatte03]

🚨 Severity: CRITICAL
💡 Vulnerability: The MERIDIAN_OBSERVABILITY_SQLITE_JOURNAL_MODE environment variable was injected directly into an f-string for PRAGMA journal_mode without validation.
🎯 Impact: While the current usage requires configuring an environment variable prior to runtime, an unvalidated value poses a SQL injection risk since PRAGMA statements do not support standard parameter binding, potentially enabling arbitrary query execution against the observability database.
🔧 Fix: Implemented a strict allowlist validation against valid SQLite journal modes (DELETE, TRUNCATE, PERSIST, MEMORY, WAL, OFF). Any unrecognized value correctly falls back to WAL.
✅ Verification: The test suite for meridian_platform passes, verifying standard observability functionality.

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PR created automatically by Jules for task 16804641921735971563 started by @mapleleaflatte03

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