Warn operators when invites are log-only (EMAIL_PROVIDER=log)

## Summary

Default `EMAIL_PROVIDER=log` does not send mail; the accept URL is logged/returned. Easy to miss on a VPS if someone leaves the default.

## Context

- `src/voxforge/infrastructure/email/invite_mailer.py` (`invite_email_log_only`)
- Dashboard invite UI
- `tests/unit/test_invite_email.py`

## Acceptance criteria

- [ ] Invite-create API includes a clear warning when email is not delivered
- [ ] Dashboard shows that warning after create
- [ ] Unit test for log provider
- [ ] Known-limitations still accurate

Do not send real email in PR CI.

Start with src/voxforge/infrastructure/email/invite_mailer.py and the invite_email_log_only path, then trace invite creation into the dashboard invite UI. Run tests/unit/test_invite_email.py first and add coverage for the log provider. Done means the API and dashboard clearly warn that email was not delivered, known-limitations remains accurate, and PR CI sends no real email.

Written by the indexing model from the issue text.