Fix subscriber count response handling

[ma2za]

Summary

• support Substack's current subscribers list response
• preserve compatibility with the legacy subscriberCount response
• add regression tests for both response shapes

Root cause

The publication launch checklist endpoint no longer returns a top-level subscriberCount for this account. It now returns subscriber records in a subscribers list, causing substack status to raise KeyError.

Impact

substack status reports the subscriber count with both the current and legacy API response formats.

Validation

• python -m pytest tests/substack/test_api.py tests/substack/test_cli_operations.py -q: 23 passed, 5 skipped
• live substack status: completed successfully
• commit hooks: Check Yaml, Fix End of Files, Trim Trailing Whitespace, black, and isort passed

The full suite remains blocked by the unrelated missing mdit_py_plugins.subscript module in the local environment.