feat: add a host-provided outbound email transport

Ekklesia could authenticate people and pair devices but could not send a
single message. Better Auth has been configured with emailAndPassword
since ADR 0011 and neither of its mail hooks — sendResetPassword and
emailVerification.sendVerificationEmail — was ever implemented, so email
verification, password recovery, and invitations were all blocked on the
same missing thing.

That missing thing is a transport, not a Google Workspace integration.
Email is now a host-provided infrastructure service with a provider seam,
shaped exactly like object storage: feature code says "send this
transactional message" and the provider owns the host, port, TLS,
credentials, connection behaviour, and every transport error. No
provider's hostname appears anywhere in Core, and a test asserts it.
Google Workspace SMTP Relay is the expected first deployment and a
compatibility target — the same variables address Microsoft 365, a
transactional provider, or a local Postfix.

The contract is deliberately tiny: one recipient or a small bounded list,
a subject, text and/or HTML, an optional reply-to, and nothing else. There
is no `from` field at all — the sender is host configuration, so no
feature and no future module can choose who mail appears to come from,
which is how a platform otherwise acquires an impersonation primitive.
Control characters are refused rather than stripped in addresses, display
names, subjects, and reply-to values; writing that rule down was not
enough, because the first implementation trimmed an address before
checking it and silently accepted a trailing carriage return until a test
caught it.

Security modes are named rather than inferred from a boolean and a port,
which is genuinely ambiguous: `secure: false` on 587 means "connect in the
clear and hope STARTTLS is offered". `starttls` sets requireTLS so a
stripped upgrade fails the send instead of continuing in plaintext,
production refuses `plaintext` outright, and certificate verification has
no off switch. Credentials are optional as a pair, because a relay
authorized by source IP has neither and requiring them would make the
intended production arrangement unconfigurable; half a pair fails loudly.

Connection, greeting, and socket timeouts are all explicit — the
object-storage pass already showed what a network client on library
defaults does to a request. Nothing retries: a failure after the message
data has gone out is ambiguous, and retrying it turns one password-reset
request into several emails, so this is a single bounded attempt and
deliberately not an exactly-once promise.

Email stays optional. An installation with no SMTP host is a complete
deployment, nothing sends at startup, and constructing the transport opens
no socket. There is no POST /api/v1/send-email and no route that touches
the provider — an authenticated send endpoint is an open relay with extra
steps. Modules get nothing either; services are added one at a time when
something real needs one, and nothing does yet.

Proven against a real in-process SMTP server rather than a mock: delivery
with the host's sender on the wire, authentication and its sanitized
refusal, a rejected mailbox distinguished from an outage, a refused
connection, a silent server hitting its deadline, and a starttls transport
refusing to continue when the upgrade is not offered. No test sends real
email and no live credential exists in this repository or in CI.

Co-Authored-By: Claude Opus 5 <[email protected]>

docs/development/api.md

@@ -863,6 +863,20 @@ instead, which now carries `viewer: {kind: "authenticated", userId} | {kind: "de
 `SameSite=Strict`, a display must be served from the same origin as the API — which the platform's
 own `/display/<module-id>` route is.
 
+## Outbound email
+
+There is **no HTTP API for it**, deliberately
+([ADR 0037](../architecture/decisions/0037-host-provided-outbound-email.md)). Email is a runtime
+capability trusted server-side features call, not a resource the platform exposes: an authenticated
+send endpoint is an open relay with extra steps, so no route exists and no route file imports the
+mail module. A test asserts both.
+
+Host configuration is `EKKLESIA_SMTP_HOST`, `EKKLESIA_SMTP_PORT`, `EKKLESIA_SMTP_SECURITY`,
+`EKKLESIA_SMTP_USERNAME`/`EKKLESIA_SMTP_PASSWORD` (optional as a pair), `EKKLESIA_SMTP_FROM_ADDRESS`,
+and `EKKLESIA_SMTP_FROM_NAME`. Absent `EKKLESIA_SMTP_HOST` means this installation sends no email,
+which is a complete deployment. None of these values ever reaches a module, a browser bundle, a
+diagnostic, or an API response. See [Outbound email](email.md).
+
 ## Module HTTP routes
 
 An installed module's routes (declared via `http.route(...)` in its `register` hook — see