Follow-up to the #99641 salvage:
- One module-level _normalize_security() (ssl/tls/implicit -> tls, starttls,
plain/none -> plain; unknown -> WARNING + secure default) replaces the three
copies of the alias set; _connect_imap/_connect_smtp/_standalone_send all
compare against the canonical value. Unknown modes no longer raise.
- _tls_context(verify, host) is module-level and shared by all sites; when
verification is disabled for a non-loopback host it logs a WARNING.
- _esecret_bool: an unset/empty env var now yields the caller's default
(previously is_truthy_value('') returned False, silently disabling TLS
verification whenever EMAIL_*_TLS_VERIFY was unset).
- Documented surface is platforms.email.extra.{imap,smtp}_security and
{imap,smtp}_tls_verify in config.yaml; env vars remain an internal bridge
and are NOT added to plugin.yaml (optional_env feeds hermes setup prompts).
- Docs: Proton Mail Bridge / local relays recipe in user-guide/messaging/email.md.
- Tests: starttls builds IMAP4 then .starttls(); unknown mode falls back to
tls/starttls with verification still on.
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