refactor(server): fold the notifications domain out of services/

Four files taken apart rather than moved, because they only worked as a knot.

The mailer becomes its own leaf module. AuthService sends the password-reset
mail while NotificationsModule imports AuthModule for the demo gate in
NotificationsMcp - a mailer inside the notifications domain would have closed a
hard cycle that only a forwardRef could open again. Webhook and ntfy stay with
notifications; auth has no business importing them.

The channel registry stays a module singleton and does NOT become a provider.
PluginRuntimeService pushes its channel getter in at onModuleInit, and
notifications.bridge.ts builds its own NotificationsService for the scheduler
crons and the memories senders. A provider would hand the bridge a second, empty
registry - every plugin channel would then go quiet with no error anywhere.
CHOVR-015 drives that seam end to end: a channel published through the runtime,
delivered through the bridge. Nothing pinned it before; the existing cases all
set the source themselves.

The built-ins no longer self-register by side-effect import. They used to exist
only because notificationPreferencesService happened to `import
'./notifications/builtins'`; removing that line would have silenced email,
webhook and ntfy without a failure. They are built from the injected transports
and registered from NotificationsService's constructor, so both instances have
them.

No cached nodemailer transport: MailerService.createTransport re-reads the SMTP
config and skipTlsVerify per send, exactly as the three inline call sites did,
so an admin changing them does not have to restart.

Preferences become a provider over DatabaseService. Eight places built
NotificationsService by hand, so a tests/helpers/notifications.ts factory keeps
the next constructor parameter a one-file diff instead of an eight-file one.

server/tests/helpers/mcp-test-controllers.ts

@@ -47,6 +47,7 @@ import { VacayMcp } from '../../src/nest/vacay/vacay.mcp';
 import { VacayService } from '../../src/nest/vacay/vacay.service';
 import { RealtimeService } from '../../src/nest/realtime/realtime.service';
 import { QueryHelpersService } from '../../src/nest/query-helpers/query-helpers.service';
+import { makeNotificationsService, makeNotificationPreferencesService } from './notifications';
 
 /**
  * Hand-wired counterpart of the boot-time discovery in McpRegistryService,
@@ -104,7 +105,7 @@ export function createMcpTestRegistry(): McpRegistry {
       new CollectionsMcp(new CollectionsService(dbService, permissionsService, realtimeService), dbService, authService),
       new TransitMcp(new TransitService(), daysService, reservationsService, dbService, authService),
       new AtlasMcp(new AtlasService(dbService)),
-      new NotificationsMcp(new NotificationsService(dbService, realtimeService), authService),
+      new NotificationsMcp(makeNotificationsService(dbService, realtimeService), authService),
     ],
     { accessPolicy: trekMcpAccessPolicy, validateAccess: trekMcpValidateAccess },
   );