[Feature] Allow the background log path to be specified

[cancan101]

What

comfy launch --background writes ComfyUI's output to
<workspace>/user/comfyui_<port>.log, and there is currently no way to ask for a
different path. A --log-file option on launch, or an env var honoured by
background_log_path(), would let a supervising process keep that log wherever it
keeps its others.

Why

comfy-cli is increasingly something else starts — a project launcher, a dev
script, a systemd unit. Those supervisors usually have one convention for where
their managed services log (/tmp/<name>.log, /var/log/<name>/, a run
directory), and ComfyUI is the one participant whose log lands somewhere else, in
a location that also moves with the workspace.

In our case (a launcher that starts piper / ollama / a Chatterbox bridge / ComfyUI
side by side) every other helper announces (logs: /tmp/<name>.log) on startup
and points back there on failure. ComfyUI can't, so we symlink /tmp/comfyui.log
at comfy-cli's file instead. That works fine — this is a convenience request, not
a defect report.

Current state, for reference

• launch exposes --background and --frontend-pr; no log option.
• background_launch() computes log_path = background_log_path(port)
  unconditionally.
• The background_log config key looks like it might be an input, but it's
  written by comfy-cli after launch and read back only by comfy logs to locate
  the file.

Suggested shape

Either is fine; the env var alone would cover the supervisor case:

def background_log_path(port, workspace: str | None = None) -> str:
    override = os.environ.get("COMFY_BACKGROUND_LOG")
    if override:
        return os.path.abspath(override)
    ...

One thing worth preserving

_open_log_for_write() opens with O_NOFOLLOW so a pre-placed symlink in
<workspace>/user can't be used to clobber its target — a good defence, and an
override path should keep it rather than special-casing "trusted" locations. Worth
saying explicitly in the docs for such an option: the override must be a real
file path, not a symlink, or the launch fails with ELOOP.

That behaviour is also why the workaround has to point the other way (a symlink
at comfy-cli's file, never the workspace path symlinked outward) — which is
easy to get backwards, and might deserve a line in the comfy logs docs
regardless of whether this request is taken up.