Add deploy preflight checks for email configuration

[davegaeddert]

Email misconfiguration is a silent failure. The console, preview, and locmem backends all return a success count from send_messages() — nothing raises, so no exception tracker sees the loss. Anything that depends on email (password resets via plain.passwords, login links via plain.loginlink) just stops working with no signal.

Adds two deploy=True preflight checks in plain.email, following the security.debug pattern for "dev convenience left on in production."

Checks

Check	Result id	Flags	Severity
email.backend	email.backend_does_not_deliver	EMAIL_BACKEND set to the console, preview, or in-memory backend	Error
email.smtp_host	email.smtp_host_empty	SMTP backend with an empty EMAIL_HOST	Error
email.smtp_host	email.smtp_host_is_default	SMTP backend with EMAIL_HOST still at its default "localhost"	Warning

email.smtp_host exists so email.backend doesn't hand out false confidence: the more common production failure isn't the console backend left on, it's a correct-looking SMTP config that was never pointed at a mail server. An empty host is an error because smtplib skips connecting when the host is falsy, so starttls() raises SMTPServerDisconnected on the first send. The default "localhost" is only a warning — a mail relay on the same host is a legitimate setup.

Deployments that intentionally send no email silence by result id; the README documents both ids.

Notes

• Gating is free: preflight autodiscovery imports per installed package, so the checks only exist when plain.email is in INSTALLED_PACKAGES.
• Backend import paths move to plain/email/backends/__init__.py, so the toolbar, the mailoutbox fixture, and the checks share one definition instead of three copies of the same string.
• example/app/settings.py uses the preview backend, so plain preflight --deploy there now errors. That's the check working as intended; no CI job passes --deploy.

Testing

• 8 new tests in plain-email/tests/public/test_preflight.py (22 pass in the package).
• Verified end-to-end: plain preflight --deploy in the test app reports email.backend_does_not_deliver against its console backend, and a run without --deploy skips both checks.
• ruff check / ruff format --check / ./scripts/type-check plain-email all clean.

The full ./scripts/test and ./scripts/fix runs could not complete in this environment — no Postgres available, and the oxlint/oxfmt download is blocked by the sandbox proxy. Python linting, formatting, type checks, and the plain-email suite were all run directly instead.

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