fix(imap): deduplicate scan results on insertion

[devprashant19]

Description

This PR fixes a bug in backend/email_connectors/imap_store.py where IMAP scan results were inserted blindly on every scan cycle. Because there was no deduplication, the same emails were re-inserted every time the inbox was scanned, causing the database table to grow indefinitely (~144,000 redundant rows per month per user on a 15-minute interval).
Closes #223

Changes

• Updated save_scan_results() in imap_store.py to first query the database for existing message_ids.
• Filtered the incoming scanned_emails list to only execute INSERT statements for emails that are not already present in the database.
• This approach safely prevents duplicates without requiring complex schema migrations on the deployed SQLite database.

Related Issues

• Resolves the unbounded IMAP scan results duplication bug.

Type of Change

• Security fix
• Bug fix
• New feature
• Performance improvement

Pre-submission Checklist

• Verified that repeated scans do not result in duplicate records in imap_scan_results.
• Ensured new emails are still successfully inserted.

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