Run Tandoor as a rootless Podman Quadlet

Replaces the docker_container role with a Quadlet systemd user unit, and moves
state from /var/lib/tandoor to ~/podman-data/tandoor. Tandoor's entrypoint
starts as root and drops privileges, but root is what ends up owning the data,
so keep-id maps container root onto the connecting user and everything stays
owned by tanshu rather than a subuid.

The play does not run the rootless-Podman prerequisites: they are the shared
task file at caddy/roles/podman, and a host that has run caddy/playbook.yml has
them. Both integrations go through the shared includes rather than local copies
-- caddy/roles/register for the network and the Caddy drop-in, postgres/roles/
attach for the database -- so this app writes the same drop-in bytes as every
other service and a new service never means editing Caddy or Postgres.

Three ordering details are load-bearing:

- The register include runs in pre_tasks, before the unit is started. Quadlet
  fails unit *generation*, not merely startup, when a referenced .network file
  does not exist yet, so starting first reports "unit not found".
- roles/postgres flushes handlers immediately after attaching. The restart that
  joins Postgres to this network is a handler, so by default it runs at the end
  of the play -- after the app has already started and failed to resolve `db`.
- The role waits for a real HTTP response from inside the container instead of
  trusting systemctl is-active. The entrypoint runs Django migrations before
  gunicorn answers, and until it does the image's own nginx returns 502, which
  Caddy relays verbatim -- complete with `server: nginx`, which reads as a stray
  reverse proxy that is not actually there.

Also drops the dead nginx.conf.j2 and the network role, both superseded by the
shared includes, and moves .env into the role's templates/ as tandoor.env.j2,
rendered at mode 0600 and read through EnvironmentFile=.

tandoor/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