Avoid eager FTP connection when importing SINAN helpers

[YuF-9468]

Closes #242

Summary

• defer creation of the SINAN FTP-backed client until one of the public helper functions is actually called
• cache the loaded client so repeated calls keep the previous behavior without reconnecting on every use
• add a regression test that verifies importing pysus.online_data.SINAN no longer calls SINAN.load() eagerly

Testing

• uv venv .venv
• source .venv/bin/activate
• uv pip install -e .
• python -m unittest pysus.tests.test_sinan_online_data -v

[fccoelho]

Looks good to me.