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/Dockerfile

@@ -150,7 +150,10 @@ ENV FLASK_APP=app.py
 # Thread caps. onnxruntime (FastEmbed) ignores OMP_NUM_THREADS and sizes its
 # pool to the host's core count, which a CPU-limited container still reports;
 # EMBEDDINGS_THREADS pins it the way OMP_NUM_THREADS pinned torch before.
-ENV MALLOC_ARENA_MAX=2 \
+# The image ships no checkout, so pin the data home (inputs, indexes, .env)
+# instead of depending on the working directory.
+ENV DOCSGPT_HOME=/app \
+    MALLOC_ARENA_MAX=2 \
     OMP_NUM_THREADS=4 \
     MKL_NUM_THREADS=4 \
     OPENBLAS_NUM_THREADS=4 \