perf: fix N+1 query in email scanner deduplication

- Pre-fetch all existing scanned emails using an `in` query.
- Use a `Set` for O(1) existence checks.
- This eliminates the N+1 database queries performed when iterating over fetched messages.

Co-authored-by: 5queezer <34570565+5queezer@users.noreply.github.com>

Author: google-labs-jules[bot]