A provider declares what it serves: mail's domain, the identity provider's issuer (issue 173)

Consumers read `${bound:smtp:domain}` and `${bound:oidc-client:issuer}`, and both keys reached
them only because a module's settings were laid over everything it served. Issue 173 stops that: a
setting overrides a key a served fact declares and adds none. So the two providers declare the keys
their consumers read, as the operator's value (`${setting:…}`, ADR 0155), and the setting that
already carries each fills it. Nothing a consumer reads changes.

Merges first: under the controller that still merges settings over served facts this is the same
value, and the controller that stops merging (mesh-controller, feat/the-mesh-places-its-own-files)
needs these declared before it rolls out.

modules/mailu/module.json

@@ -552,7 +552,8 @@
   ],
   "serves": {
     "smtp": {
-      "port": 587
+      "port": 587,
+      "domain": "${setting:domain}"
     }
   },
   "receives": {