Run Valkey as a rootless Podman Quadlet

Replaces the docker_container playbook, which published 127.0.0.1:6379 and kept
its /data in an anonymous Docker volume -- the one the audit flagged because no
backup script can see it (docs/podman-compat-audit.md 3.1). Valkey becomes the
base Quadlet unit, the same shape as Postgres: it knows nothing about its
clients, and a service attaches itself by dropping a file into
valkey.container.d/ via valkey/roles/attach. Nothing on the host needs 6379, so
there is no PublishPort at all rather than a narrower one.

The name stays `valkey` for the directory, container, unit and Podman network,
and only the alias clients connect on is `redis`: Nextcloud's config.php
(written by the image from REDIS_HOST=redis), outline and overleaf reach it as
`redis`, while paperless-ngx and yamtrack use the container name. Renaming would
touch every REDIS_HOST for no gain.

The uid/gid mapping is the detail worth spelling out, because the audit only
records the uid. The Alpine image's `valkey` account is 999:1000, not 999:999 --
Alpine already has a gid 999, and the Dockerfile says so before creating the
group with the next id. 999 is the uid the anonymous volume was owned by, so the
uid matches what was already on disk, but mapping only the uid would leave every
file valkey-server writes, dump.rdb included, group-owned by a subgid. Both ids
map onto the connection user, so ~/podman-data/valkey stays tanshu:tanshu.

Persistence stays on deliberately, and the mount with it. The contents are
disposable -- nothing is copied off monoco, and a cold start is survivable -- but
this is not purely a cache: Nextcloud keeps its PHP sessions and
memcache.locking in it, and Yamtrack uses it as its Celery broker. With RDB on, a
rebuild is a graceful stop that writes a final snapshot on SIGTERM that the next
container loads, so a redeploy does not log every Nextcloud user out or discard
queued tasks. Tmpfs=/data with `save ""` would remove the mount and the whole
keep-id mapping with it, and is the right shape for a pure cache, but it would
make every redeploy exactly that.

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.

Two ordering details are load-bearing:

- The attach include asserts that <name>.network exists before writing a drop-in
  that names it. Quadlet rejects the whole valkey unit, with an error that never
  mentions the drop-in, when a referenced .network file is missing.
- A service that attaches flushes handlers before starting the app. The restart
  that joins Valkey to the new network is a handler, so by default it runs at the
  end of the play, after the app has already come up and failed to resolve
  `redis`. nextcloud/roles/valkey is the working example.

The play waits on `valkey-cli ping` inside the container rather than trusting
systemctl is-active. Restart=always reports active even while the container
crash-loops, which is exactly what a wrong keep-id looks like.

valkey/playbook.yml

@@ -1,21 +1,32 @@
 ---
-- hosts: all
-  become: true
+# Deploys Valkey as a rootless Podman Quadlet unit.
+#
+# Connects as the container user and runs everything as that user: the Quadlet
+# units belong to it, so the connection's PAM session is what makes
+# `systemctl --user` work without any environment juggling.
+#
+# gather_facts is required: podman_user and the home directory come from the
+# user_id / user_dir facts.
+#
+# The rootless-Podman prerequisites (installing podman, enabling lingering, the
+# low-port sysctl, creating the Quadlet directory) are deliberately NOT applied
+# here. They are the shared task file at caddy/roles/podman/tasks/main.yaml, and
+# a host that has already run caddy/playbook.yml has them:
+#
+#   - name: Rootless Podman prerequisites
+#     ansible.builtin.include_tasks: "{{ playbook_dir }}/../caddy/roles/podman/tasks/main.yaml"
+#
+# The variables those tasks need are duplicated at the top of vars/default.yml,
+# so this play stands on its own.
+#
+# Run with: ansible-playbook -u tanshu valkey/playbook.yml
+- name: Deploy Valkey as a rootless Podman Quadlet
+  hosts: emily
+  gather_facts: true
   vars_files:
     - vars/default.yml
 
-  tasks:
-    - name: Pull default valkey image
-      docker_image:
-        name: "{{ container_image }}"
-        source: pull
-        force_source: yes
-
-    - name: Create valkey container
-      docker_container:
-        name: "{{ container_name }}"
-        image: "{{ container_image }}"
-        state: started
-        restart_policy: "unless-stopped"
-        published_ports:
-          - 127.0.0.1:6379:6379
+  roles:
+    # The base Valkey unit. It does NOT know which app networks it serves.
+    # Each service attaches itself via valkey/roles/attach.
+    - valkey