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/roles/attach/tasks/main.yaml

@@ -0,0 +1,94 @@
+---
+# Attach a service's network to Valkey.
+#
+# Valkey is the *base* unit and knows nothing about its clients, the same way
+# Caddy knows nothing about its upstreams. A service that needs a cache adds
+# itself by dropping files next to valkey.container, and Quadlet merges them in.
+#
+# Include this from a service playbook:
+#
+#   - name: Attach this service to Valkey
+#     ansible.builtin.include_tasks: "{{ playbook_dir }}/../valkey/roles/attach/tasks/main.yaml"
+#     vars:
+#       network_name: "{{ network_name }}"
+#
+# The including play must:
+#
+#   1. have created {{ network_name }}.network already — caddy/roles/register
+#      does that, and every service here that needs a cache is also behind
+#      Caddy, so its register include comes first;
+#   2. run the `podman` role (or include its task file and define the same four
+#      vars), for podman_user and quadlet_dir;
+#   3. define a "Restart Valkey" handler — see
+#      roles/valkey/handlers/main.yaml for the exact snippet; and
+#   4. flush handlers before starting the app, on a first attach. This include
+#      notifies that restart, so by default it runs at the end of the play —
+#      after the app has already started and failed to reach `redis`. A
+#      `meta: flush_handlers` immediately after this include is enough; see
+#      nextcloud/roles/valkey for the working example.
+#
+# It runs as the connection user, which must be the rootless podman_user.
+#
+# A service may instead write these two files from its own role; the bytes have
+# to match this file exactly, or the two will overwrite each other on every run.
+
+- name: "Check that the {{ network_name }} network unit exists"
+  ansible.builtin.stat:
+    path: "{{ quadlet_dir }}/{{ network_name }}.network"
+  register: valkey_attach_network
+
+# A drop-in naming a .network file that does not exist makes Quadlet fail to
+# generate the whole valkey unit, with a message that does not mention the
+# drop-in at all. Fail here instead, where the cause is obvious.
+- name: "Fail if {{ network_name }} has no network unit"
+  ansible.builtin.assert:
+    that: valkey_attach_network.stat.exists
+    fail_msg: >-
+      {{ quadlet_dir }}/{{ network_name }}.network does not exist, so attaching
+      it to Valkey would break the valkey unit. Create the network first
+      (caddy/roles/register does it), then include this file.
+
+- name: Ensure the Valkey Quadlet drop-in directory exists
+  ansible.builtin.file:
+    path: "{{ quadlet_dir }}/{{ valkey_container | default('valkey') }}.container.d"
+    state: directory
+    owner: "{{ podman_user }}"
+    group: "{{ podman_user }}"
+    mode: '0755'
+
+- name: "Attach {{ network_name }} to Valkey"
+  ansible.builtin.copy:
+    dest: >-
+      {{ quadlet_dir }}/{{ valkey_container | default('valkey') }}.container.d/50-{{ network_name }}.conf
+    owner: "{{ podman_user }}"
+    group: "{{ podman_user }}"
+    mode: '0644'
+    content: |
+      # Added by the {{ network_name }} playbook. Do not edit by hand.
+      #
+      # Deleting this file removes the network from {{ valkey_container | default('valkey') }} on
+      # the next daemon-reload; nothing else changes.
+      [Container]
+      Network={{ network_name }}.network
+  notify: Restart Valkey
+
+# NetworkAlias= applies to every network the container joins, which is exactly
+# right for a hub: this repo's cache clients reach Valkey as `redis` (nextcloud,
+# which writes REDIS_HOST=redis into its config, plus outline and overleaf),
+# while the rest use the container name. It is a fixed value in one shared file,
+# not a per-service option, so the same bytes are written no matter how many
+# services attach — two services writing their own aliases would make Quadlet
+# pass --network-alias twice.
+- name: "Give {{ valkey_container | default('valkey') }} the shared 'redis' alias"
+  ansible.builtin.copy:
+    dest: >-
+      {{ quadlet_dir }}/{{ valkey_container | default('valkey') }}.container.d/10-network-aliases.conf
+    owner: "{{ podman_user }}"
+    group: "{{ podman_user }}"
+    mode: '0644'
+    content: |
+      # Shared container aliases for {{ valkey_container | default('valkey') }}.
+      # Managed by Ansible (the service playbooks that use it). Do not edit.
+      [Container]
+      NetworkAlias=redis
+  notify: Restart Valkey