fix(cli): review pass on the docsgpt command

- The image pins DOCSGPT_HOME=/app: it ships no checkout, so the data home
  no longer depends on the working directory.
- api, worker, beat and migrate print the data home and env file they
  resolved, so an API and a worker started from different directories show
  it.
- The worker passes -Q only when asked; a bare worker consumes every
  configured queue, which honours EMBEDDINGS_QUEUE and DOCUMENT_PARSE_QUEUE.
- The worker runs through celery.start and returns its exit code; click
  usage errors print usage and exit 2 instead of a traceback.
- Windows: solo pool and no embedded scheduler (celery rejects -B there),
  with a pointer to the new `docsgpt beat` command, which runs the
  scheduler on its own.
- prefetch_models and verify_offline parse their arguments, so --help is
  help rather than a model name.
- A DOCSGPT_ENV_FILE that is not a file raises instead of booting with
  defaults.

docsgpt/core/paths.py

@@ -43,5 +43,8 @@ def env_file() -> Path:
     """The ``.env`` file settings load: ``DOCSGPT_ENV_FILE``, else ``<home>/.env``."""
     configured = os.environ.get(ENV_FILE_ENV)
     if configured:
-        return Path(configured).expanduser()
+        path = Path(configured).expanduser()
+        if not path.is_file():
+            raise FileNotFoundError(f"{ENV_FILE_ENV} is set to {path}, which is not a file")
+        return path
     return home_dir() / ".env"