Document invite email (Resend/SMTP) in the deployment guide

## Task

`InviteEmailSender` supports `log`, `resend`, and `smtp`. `docs/CONFIGURATION.md` lists the vars; `docs/deployment/guide.md` does not walk operators through turning on real invite email for pilots.

## Context

- `src/voxforge/infrastructure/email/invite_mailer.py`
- `docs/CONFIGURATION.md` (`EMAIL_PROVIDER`, `RESEND_API_KEY`)
- `docs/release/known-limitations.md` (log mode)
- `docs/deployment/verification-checklist.md`

## Acceptance criteria

- [ ] Deployment guide section: log vs resend vs smtp
- [ ] Invite-only + `REGISTRATION_ENABLED=false` called out
- [ ] Checklist item for “invite email actually arrives”
- [ ] No secrets in examples

Start with docs/deployment/guide.md and compare its configuration guidance with docs/CONFIGURATION.md and src/voxforge/infrastructure/email/invite_mailer.py. Review docs/release/known-limitations.md for log mode and docs/deployment/verification-checklist.md for the appropriate checklist location. Done means the guide covers log, resend, and smtp, explains invite-only registration, uses no secrets, and checks that an invite email arrives.

Written by the indexing model from the issue text.