Make the Nextcloud playbook standalone: first run installs, later runs update

The playbook is meant to be usable on its own, on any host, the way every other
one here is: the first run creates the container, and every run after that
converges it. It had picked up a migration shape instead -- a readme that assumed
a webroot copied in from monoco, a warning that nagged when config.php was
missing, and, worse, readiness gates that only passed for an instance that was
already installed.

That last one was a real bug for the intended use. On a host with no webroot, the
image initialises the code, writes no config.php because it has no admin
credentials to install with, and serves the setup wizard -- so `occ status` never
reports installed: true, and the play would have spent its whole retry budget and
then failed on a first run that was going exactly as designed.

What replaces it:

- The readiness gate is Apache answering status.php inside the container, which
  is Nextcloud's own endpoint and reports `installed` either way. It is the one
  thing `is-active` cannot tell us, and it passes for a wizard that has not been
  completed.
- config.php's presence then decides the report: wait for maintenance mode to
  clear and print `occ status` for an instance, or say what is left for a human to
  do for a fresh one -- and succeed. The two optional credentials in
  vars/default.yml install unattended instead, in which case the same endpoints
  come back installed and the wizard note never appears.
- The app creates its own database. Postgres is a shared unit with one default
  database and nothing else in this repo knows this app's database name, so a
  fresh cluster previously reached the installer and failed on "database
  nextcloud does not exist". It is a lookup and a `createdb`, a no-op afterwards,
  and it fails with the instruction to run postgres/playbook.yml if the cluster
  is not there rather than with a bare psql error.
- Updates. An update in a Quadlet deployment is "pull, then recreate": the unit
  names a tag, `podman run` reuses the local image, and nothing fetches a newer
  one. The role pulls on every run and compares the image ID either side, so an
  unchanged tag is not a change and a running site is not restarted for nothing.
- Handlers are flushed before verification, so a queued recreate -- new image,
  changed unit, changed environment -- happens before the checks rather than after
  them, and the checks describe what will keep running.

Removed: the entire monoco migration section and its copy commands, the
config.php warning that existed only to serve it, the host-specific notes, and the
narrative comments about this play's first runs. References to the audit and
migration planning docs go with them; the ownership reasoning is stated inline,
since it does not depend on where an instance came from. `hosts:` is now the only
host-specific thing in the directory.

The readme is rewritten around the two paths it now has -- "First run" and
"Updates" -- with the deploy order, the session-file trap and the debugging
commands kept.

nextcloud/roles/caddy/tasks/main.yaml

@@ -10,9 +10,8 @@
 # when the Caddyfile does not exist yet, on a host where the Caddy playbook has
 # not run.
 #
-# The marker is unchanged from the Docker-era role, so an existing
-# cloud.hopsngrains.com block written on monoco is replaced in place rather than
-# duplicated.
+# The marker is stable, so re-runs replace this site's block in place rather than
+# appending a second copy of it.
 - name: "Add the {{ host }} site block to the Caddyfile"
   ansible.builtin.blockinfile:
     path: "{{ caddyfile_path }}"