fix: the image brings its own PID 1 instead of asking for init: true (4295)

Operator, 2026-09-23: *"it is out of the norm to require this init call we
need to fix this."* Correct, and it is the same mistake as declaring the
healthcheck per service — the image needing a deployment to remember a flag
before it behaves correctly.

PID 1 carries a duty no other process has: every orphaned process in the
container reparents to it and must be reaped or it stays a zombie holding a
PID slot. This app makes orphans in normal operation — six service modules
shell out (gallery_dl, thumbnailer, backup_service, external_fetch,
download_service, download_backends) and celery's prefork pool forks children
that spawn them.

Whatever the role, something that is not an init ends up as PID 1:
supervisord for `all`, hypercorn for `web`, celery for a worker. `init: true`
covered that, and cost correctness the moment it was forgotten or silently
dropped — an older Swarm, a plain `docker run`, a compose file someone
copied. No signal either way.

So tini goes in the image and is the ENTRYPOINT. `docker run <image>` is
correct on its own now, `init: true` comes out of docker-compose.single.yml,
and nothing downstream has to know. The smoke asserts /proc/1/comm is tini,
read from /proc because the runtime stage installs no `ps`.

## A correction to what I told the operator

I justified `init: true` by saying supervisord "has no idea about orphans it
never started". That is very likely wrong: supervisord's reaper calls
waitpid(-1) and logs "reaped unknown pid" for children it did not spawn, so
it does reap orphans. I asserted the mechanism without checking it, and could
not check it here — `supervisor` is not installed in this environment.

It does not change this commit. tini is correct whichever way that lands, and
it covers the single-role containers too, where celery or hypercorn is PID 1
and the subprocess-spawning is heaviest. But the reason I gave was not a
verified one and should not have been stated as fact.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LVjrnpQjRgHdvq95rASoiR

docker-compose.single.yml

@@ -63,12 +63,6 @@ services:
     # under supervisord — is what the image does by default, and supervisord's
     # config is generated from the application's own lane table so the two
     # cannot disagree. `command: ["all"]` still works and means the same thing.
-    # tini as PID 1, in front of supervisord. supervisord reaps its own
-    # children, but a container's PID 1 also inherits orphans from anywhere
-    # below — celery's prefork pool and gallery-dl's subprocesses both make
-    # them. Without this they accumulate as zombies for the life of the
-    # container.
-    init: true
     # Sized to the SLOWEST lane, not the average. maintenance_long runs DB
     # backups, library audits and translation backfill, and gets 180s to
     # finish a chunk; the lanes stop in parallel, so this covers the max