infra(mailpit): use nc -z healthcheck instead of wget

The mailpit service healthcheck previously assumed `wget` ships in
the axllent/mailpit image. That's true for v1.29.7 but is not part
of the image's contract — a future Alpine slim-down could drop wget
and silently disable the healthcheck. Switched to BusyBox `nc -z
localhost 8025`, which is a TCP-port open check with no dependency
beyond BusyBox itself.

Verified inside axllent/mailpit:v1.29.7 that `nc` is present
(/usr/bin/nc, BusyBox v1.37.0) and that the proposed command
returns 0 against an open port and non-zero against a closed one.
Compose still parses with `--profile staging`. Addresses @tobi's
round-2 suggestion on PR #499.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

docker-compose.prod.yml

@@ -103,7 +103,11 @@ services:
     networks:
       - archive-net
     healthcheck:
-      test: ["CMD-SHELL", "wget -qO- http://localhost:8025/api/v1/info >/dev/null 2>&1 || exit 1"]
+      # TCP-port open check via BusyBox `nc`. The previous wget-based probe
+      # introduced a non-obvious binary dependency on the mailpit image; a
+      # future tag that ships without wget would silently disable the
+      # healthcheck. `nc` is part of BusyBox in the upstream image.
+      test: ["CMD-SHELL", "nc -z localhost 8025 || exit 1"]
       interval: 10s
       timeout: 5s
       retries: 5