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]>

apps/api/src/config.ts

@@ -1,8 +1,8 @@
 import {
   assertProductionDatabaseTransport,
+  createDatabaseTransportPolicy,
   type DatabaseTransportPolicy,
   type DatabaseTransportPolicyInput,
-  createDatabaseTransportPolicy,
 } from "@ekklesia/core/database";
 import {
   type CreateIdentityConfigOptions,
@@ -10,6 +10,11 @@ import {
   type IdentityConfig,
   type IdentityConfigInput,
 } from "@ekklesia/core/identity";
+import {
+  createMailProviderConfig,
+  type MailProviderConfig,
+  type MailProviderConfigInput,
+} from "@ekklesia/core/mail";
 import {
   createStorageProviderConfig,
   type StorageProviderConfig,
@@ -70,6 +75,16 @@ export interface ApiRuntimeConfig {
    * bundle, diagnostic, or API response ever sees the endpoint or the credentials.
    */
   readonly storage: StorageProviderConfig | undefined;
+  /**
+   * The validated host outbound-email provider, or `undefined` when this installation has none
+   * (ADR 0037).
+   *
+   * `undefined` is a complete, valid deployment: nothing in the platform sends email yet, and a
+   * future feature that needs to discovers a structured `MAIL_NOT_CONFIGURED` rather than crashing.
+   * These values reach Core's provider and nowhere else — no module, browser bundle, diagnostic, or
+   * API response ever sees the host or the password.
+   */
+  readonly mail: MailProviderConfig | undefined;
 }
 
 export interface ApiRuntimeConfigInput {
@@ -85,6 +100,8 @@ export interface ApiRuntimeConfigInput {
   readonly port?: unknown;
   /** Optional. Omitted, or with no endpoint, means this installation has no object storage. */
   readonly storage?: StorageProviderConfigInput;
+  /** Optional. Omitted, or with no host, means this installation sends no outbound email. */
+  readonly mail?: MailProviderConfigInput;
 }
 
 export interface CreateApiRuntimeConfigOptions {
@@ -164,12 +181,19 @@ export function createApiRuntimeConfig(
   // fails startup with a sanitized code rather than surfacing later as a confusing storage error.
   const storage = createStorageProviderConfig(input.storage, { environment });
 
+  // Validated in the same pass as everything else, so a malformed port, a half-supplied credential
+  // pair, or a production deployment that asked for plaintext SMTP fails startup with a sanitized
+  // code — rather than surfacing later as a confusing failure the first time something tries to
+  // send a password-reset email.
+  const mail = createMailProviderConfig(input.mail, { environment });
+
   return Object.freeze({
     clientAddress,
     databaseTransport,
     databaseUrl,
     host,
     identity,
+    mail,
     port,
     storage,
   });