Replace vendored docker-compose with podman-compose

podman-compose and docker-compose aren't discovered the same way by
`podman compose` - verified live (a fake-binary test reading podman's own
provider-search error output) that docker-compose is searched for by
exact path across a fixed list of CLI-plugin directories, while
podman-compose is instead looked up as a plain command on $PATH. This
package installs to /usr/local/bin/podman-compose accordingly, not under
any cli-plugins/ directory.

Unlike docker-compose (a single static Go binary), podman-compose is a
Python script with two runtime dependencies neither of which ship with
Unraid's own Python3 - PyYAML and python-dotenv, vendored here as plain
pure-Python source (no C extension build; PyYAML's own fallback handles
its optional C accelerator being absent).

Verified end-to-end on a real host: with the previous docker-compose
binary temporarily moved aside to confirm podman-compose was actually the
one invoked, `podman compose up/ps/down` ran a real compose project
correctly, including a live HTTP check against the started service.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

packages/podman-compose/README.md

@@ -0,0 +1,34 @@
+# packages/podman-compose/
+
+Pinned versions: see `PODMAN_COMPOSE_VERSION`/`PYYAML_VERSION`/
+`PYTHON_DOTENV_VERSION` in [versions.env](../../versions.env).
+
+`podman compose` (backing `webui/plugins/podman/ajax/compose.php`, the
+WebUI's Compose panel) has no compose implementation of its own — it
+needs an external "compose provider" command. This project previously
+vendored `docker/compose` (the Go CLI-plugin binary) for that role;
+this package replaces it with `podman-compose` instead.
+
+The two aren't discovered the same way — verified live against a real
+podman install (placing a fake executable and reading podman's own
+provider-search error output): `docker-compose` is searched for by exact
+path across a fixed list of CLI-plugin directories, while `podman-compose`
+is looked up as a plain command on `$PATH`. That's why this package
+installs to `/usr/local/bin/podman-compose` rather than under any
+`cli-plugins/` directory.
+
+Unlike `docker-compose`, `podman-compose` is a single Python script, not a
+compiled binary. Unraid ships Python3 itself but neither of its two
+runtime dependencies, so this package also vendors:
+
+- `PyYAML` — only the pure-Python `yaml/` package, not the `_yaml` C
+  extension (which would need libyaml plus a compiler). `yaml/__init__.py`
+  falls back gracefully when the C accelerator isn't importable, so the
+  pure-Python source is sufficient for what podman-compose needs from it.
+- `python-dotenv` — pure Python throughout, no C extensions at all.
+
+Verified end-to-end on a real Unraid host: the vendored bundle correctly
+runs `podman compose up`/`ps`/`down` against a real compose project
+(with the pre-existing `docker-compose` binary temporarily moved aside
+to confirm `podman-compose` was the one actually being invoked, not a
+leftover), including a live HTTP check against the started service.