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Also drops the dead nginx.conf.j2 and the network role, both superseded by the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/files/.env

@@ -1,8 +0,0 @@
-SECRET_KEY=7ukvcoUnuyhPK6Nds3KbwG+3zeYuJGTDyi0ohFJqMwgjrdwBxn%
-DB_ENGINE=django.db.backends.postgresql
-POSTGRES_HOST=db
-POSTGRES_PORT=5432
-POSTGRES_USER=postgres
-POSTGRES_PASSWORD=123456
-POSTGRES_DB=tandoor
-ALLOWED_HOSTS={{ host }},localhost,127.0.0.1

tandoor/files/nginx.conf.j2

@@ -1,13 +0,0 @@
-server {
-
-  listen 80;
-  server_name {{ http_host }};
-
-  location / {
-    proxy_pass http://localhost:8880;
-    proxy_set_header Host $host;
-    proxy_set_header X-Real-IP $remote_addr;
-    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
-    proxy_set_header X-Forwarded-Proto $scheme;
-  }
-}