fix(pretix): send a Host header the app accepts from its healthcheck

container_hostname is computed for sethostname(2) and clamped to 63 chars
(plugins/lookup/container_hostname.py). pretix's onion domain is 74, so the
lookup degrades to the bare entity name 'pretix' -- and container_healthcheck
forwards that value into the curl probe's Host header.

pretix gates on Host in MultiDomainMiddleware, not in ALLOWED_HOSTS (settings.py
hardcodes ALLOWED_HOSTS = ['*']). It accepts the site domain, a KnownDomain, or
one of LOCAL_HOST_NAMES = ('testserver', 'localhost'); everything else gets a
400. The probe could therefore never pass, swarm killed the task with exit 137
and the stack never converged -- Pretix 0 of run 31340965955.

A/B against pretix/standalone, same volume and same PRETIX_PRETIX_URL, probe run
by Docker itself, only the header differing:
  Host: pretix     -> curl (22) 400 x4, unhealthy streak=6
  Host: localhost  -> exit 0 x3, healthy

The Tcp flavor already hardcodes Host: localhost for the same reason; curl was
the outlier. Reusing a kernel hostname as an HTTP Host is the underlying design
bug, but defaulting the curl probe would change six roles' probes at once, so it
stays a separate change.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

roles/web-app-pretix/meta/services.yml

@@ -42,6 +42,7 @@ postgres:
 pretix:
   healthcheck:
     flavor: curl
+    hostname: localhost
   modes:
     compose:
       enabled: true