Fix bugs and code quality issues found during audit

[pvernier]

form.py:

• fetch_data: fix inconsistent return — error path now stores empty DataFrame in self.data instead of returning it; return type -> None
• fetch_data: use None (object dtype) for missing survey columns so pandas 3.0 does not reject string assignment on the empty slice
• fetch_media: fix AttributeError — media is a list at the .empty check; build the DataFrame first, then check .empty on it; also fix media.metadata -> self.media["metadata"]
• _get_survey: use is None guard instead of falsy check so an empty dict asset does not bypass the cache
• _fetch_asset: call res.json() once and reuse the result
• _split_gps_coords: fix repeat-group path using list.append(a, b) (TypeError) -> list.insert(a, b)
• remove now-unused from typing import Union

manager.py:

• _fetch_forms: fix return type annotation None -> list
• get_forms / get_form: use is None cache guard so an account with zero forms does not re-fetch on every call
• upload_media_from_local: normalise file extension to lowercase and drop the mixed-case duplicate entries (.JPGE typo, .JPG, .PNG); fix unclosed file handle by wrapping open() in a with-block
• _upload_media: remove res.status_code mutation to seed the while-loop; use while True / break pattern instead

utility.py:

• reconcile_columns_append: replace TODO/XXX placeholder docstring

tests/test_form.py:

• update test_fetch_data_api_error to check f.data instead of return value