[Enhancement] Add advisory DB freshness metadata and stale warnings

[sonukapoor]

Summary:
Adds advisory DB freshness metadata and stale-data warnings so offline users can tell when their local advisory database may need to be refreshed.

Closes #79.

What changed:

• added local advisory DB metadata storage for last sync time and source URL
• updated advisory sync to persist freshness metadata into the SQLite DB
• added offline CLI output for advisory DB freshness
• added warnings when the local advisory DB is stale or missing a sync timestamp
• added tests for metadata persistence and freshness-related CLI behavior

Notes:
This is a practical first step toward TTL-style freshness handling.

It adds built-in freshness visibility and a default stale-data signal without introducing extra configuration complexity yet.

Verification:

• npm test
• npm run build
• manual end-to-end test of sync, offline scan, metadata inspection, and stale warning behavior