Run Nextcloud as a rootless Podman Quadlet

Replaces the docker_container playbook, which bind-mounted /var/lib/nextcloud at
/var/www/html on monoco and ran cron.php from root's crontab. Nextcloud now runs
on emily behind the shared Caddy, with its database in the shared Postgres and
its cache in the shared Valkey, each attached the way every other service
attaches to them.

/var/www/html is the unusual part: it holds the code as well as the state --
version.php, core/, lib/ and apps/ alongside config/config.php, data/ and
custom_apps/ -- and the entrypoint's version check reads version.php from the
mount. That is why the migration copies the whole tree rather than only the three
irreplaceable directories (docs/backup-plan.md 2.1 lists the rest as regenerable,
and they are, but only from an image that is the same version or newer). It is
also what makes an image bump an in-place upgrade: a newer image rsyncs its code
in and runs `occ upgrade` on first start, and an older one refuses to start at
all, so the version on both hosts is worth checking before the copy.

UserNS=keep-id:uid=33,gid=33. 33 is www-data, the uid the server actually writes
as -- `docker exec nextcloud id -u` reports 0, because the entrypoint drops
privileges after starting (docs/podman-compat-audit.md 3.2). 33 is also what the
data is owned by on monoco, so the copy needs no chown pass: extracting as the
connection user is already the ownership the container expects. One file is
created by container-root and never chowned, the entrypoint's
nextcloud-init-sync.lock, so the role pre-creates it -- the entrypoint truncates
that file rather than recreating it, so its owner then never changes, and the
webroot ends up with no subuid-owned file in it.

The Valkey attach is not optional. config.php carries memcache.locking pointed at
Redis, written by the image from REDIS_HOST, and the same variable rewrites PHP's
session-handler config on every start -- so an unresolvable `redis` does not just
make things slower, it fails uploads at write time. Hence the `redis` alias
rather than the container name, and hence roles/valkey flushing handlers before
the app starts, the same as roles/postgres.

cron.php moves from root's crontab to a systemd user timer, because root cannot
`podman exec` into another user's rootless containers and there is no container
user to own a crontab (docs/podman-migration.md 8b). Unlike the Quadlet units
this pair is plain and is genuinely enabled, which is what brings it back after a
reboot; journalctl --user -u nextcloud-cron shows every run, which the crontab
never did.

Readiness is three gates, because no one of them is sufficient: is-active (which
Restart=always satisfies even while crash-looping), `occ status` reporting
installed, and status.php served over loopback by Apache rather than by the PHP
CLI. The second is followed by a wait for maintenance mode to clear rather than
an assert: maintenance mode is on for the whole of a first-start `occ upgrade`,
so asserting it off would fail a healthy deploy halfway through. The case that
never clears is a config.php copied with `maintenance => true`, which
docs/backup-plan.md 3.4 tells an operator to set before a consistent dump, and
which is invisible from outside because status.php is exempt from maintenance
mode -- so the play names it and prints the command that clears it.

Register and attach ordering is the same as the other service playbooks: register
runs in pre_tasks, because the unit references nextcloud.network and Quadlet
fails unit generation when that file does not exist, and both attaches flush
handlers before the app starts, which is also what pulls Caddy onto the network.

The Caddy snippet is unchanged, HSTS header included; only caddyfile_path moves,
from /var/lib/caddy/conf/Caddyfile to ~/podman-data/caddy/conf/Caddyfile. The
Docker-era roles/network is deleted rather than ported: it existed to attach an
already-created container to a network with the `redis` alias, and
podman_network has no connected:/appends: equivalent
(docs/podman-compat-audit.md 3.3). Nothing is published host-side; Caddy reaches
nextcloud:80 over nextcloud_net.

The rootless-Podman prerequisites are not applied here: they are the shared task
file at caddy/roles/podman, and a host that has run caddy/playbook.yml has them.
valkey/playbook.yml has to have been run first, or the first Restart Valkey
handler fails against a unit that does not exist yet.

nextcloud/roles/postgres/handlers/main.yaml

@@ -0,0 +1,17 @@
+---
+# Postgres is its own playbook (postgres/playbook.yml) and a rootless Quadlet
+# unit there. This handler runs as the connection user, so `systemctl --user`
+# works without setting XDG_RUNTIME_DIR.
+#
+# Required by postgres/roles/attach, which notifies it whenever this app's
+# drop-in changes: a new drop-in changes the generated unit, so Postgres must be
+# recreated to join the network — Quadlet has no live equivalent of
+# `podman network connect`. That is a few seconds of downtime for every attached
+# app, not just this one. See postgres/readme.md, "The trade-off".
+- name: Restart Postgres
+  ansible.builtin.systemd_service:
+    name: "{{ postgres_unit }}"
+    scope: user
+    state: restarted
+    daemon_reload: true
+  listen: Restart Postgres