Judge mailu by whether mail flows, not only by its parts

On 2026-10-14 every mailu container read healthy while postfix could not
resolve a name: inbound mail was refused with 450 and outbound mail sat in
the deferred queue for days (issue 690). The smtp container is now judged
by a tool check, mailu_health, asked inside it: postfix runs (the image's
own check, kept), no message waits deferred longer than 30 minutes, and a
public mail host resolves through the resolver postfix is given.

modules/mailu/module.json

@@ -443,7 +443,12 @@
       "name": "mailu-smtp",
       "image": "ghcr.io/mailu/postfix@sha256:e2e49f39e53b80eac9e7a2f18d9df11edeb4914fd62dbba89b3155e8e034f62e",
       "health": {
-        "kind": "runtime"
+        "kind": "tool",
+        "tool": "mailu_health",
+        "interval": "60s",
+        "timeout": "30s",
+        "looks": 3,
+        "grace": "120s"
       },
       "network": "mailu",
       "env-file": [
@@ -642,6 +647,7 @@
           "MESH_MAILU_URL": "http://127.0.0.1:${port:8080}/api/v1",
           "MESH_MAILU_API_KEY_FILE": "${dir:state}/api-token.secret",
           "MESH_MAILU_IMAP_CONTAINER": "mailu-imap",
+          "MESH_MAILU_SMTP_CONTAINER": "mailu-smtp",
           "MESH_MAILU_CONFIG_FILE": "${dir:mesh-state}/config.json",
           "MESH_RECEIVES": "${dir:grants}/mesh.json"
         }