feat(email-search): add server-side query support & fix agent silent stalls

[Katsoragi]

Summary

This PR adds server-side query support for email listings. By delegating keyword searches directly to the IMAP server (SEARCH TEXT "query"), we optimize retrieval times, prevent heavy local filtering of large folder slices, and resolve silent agent stalls when searching large mailboxes.

Target branch

• This PR targets dev, not main. All PRs land in dev; main is curated by the maintainer at each release. If your PR is on main by accident, click "Edit" on this PR and change the base.

Linked Issue

Fixes #5165

Type of Change

• Bug fix (non-breaking — fixes a confirmed issue)
• New feature (non-breaking — adds new behaviour)
• Breaking change (changes or removes existing behaviour)
• Refactor / cleanup (behaviour unchanged)
• Documentation only
• CI / tooling / configuration

Checklist

• I searched open issues and open PRs — this is not a duplicate.
• This PR targets dev
• My changes are limited to the scope described above — no unrelated refactors or whitespace changes mixed in.
• I actually ran the app (docker compose up or uvicorn app:app) and verified the change works end-to-end. Type-checks and unit tests are not enough.

How to Test

1. Use the email listing tool list_emails or backend route /api/email/list with a query parameter.
2. Observe that keyword search executes server-side on the IMAP server, returning only matching emails.

Checks Run

• Run pytest suite: pytest tests/test_imap_leak_fixes.py tests/test_imap_mailbox_quoting.py tests/test_email_owner_scope.py (All Passed).