fix: ensure mms is included in exports

[naveensingh]

Type of change(s)

• [x] Bug fix
• [ ] Feature / enhancement
• [ ] Infrastructure / tooling (CI, build, deps, tests)
• [ ] Documentation

What changed and why

• Added missing projection for SUBJECT column.
• Dropped the simple view when fetching conversation IDs.
• Dropped message count filter.

Tests performed

• I can't test it.

Closes the following issue(s)

• Maybe closes https://github.com/FossifyOrg/Messages/issues/493

Checklist

• [x] I read the contribution guidelines.
• [ ] I manually tested my changes on device/emulator (if applicable).
• [ ] I updated the "Unreleased" section in CHANGELOG.md (if applicable).
• [ ] I have self-reviewed my pull request (no typos, formatting errors, etc.).
• [ ] All checks are passing.