`chant_importer.py` field names inconsistent with CantusDB CSV exports

[kyrieb-ekat]

When importing chants via the admin interface or import_data chants, the
importer may fail or silently drop data for some manuscripts. The field names
expected by cantusdata/helpers/chant_importer.py do not consistently match
the column headers present in CantusDB's CSV exports
(https://cantusdatabase.org/sites/default/files/csv/{id}.csv).

Steps to reproduce:

1. Follow the local setup instructions in the README
2. Import a manuscript and attempt to import its chants via the admin interface
3. Observe errors or incomplete chant data depending on the manuscript

Expected behaviour: All fields present in the CantusDB CSV are correctly
parsed and stored.

Notes: This likely requires comparing the current importer field mappings
against a sample of real CantusDB CSV exports to identify which columns are
missing or misnamed. The affected manuscripts and specific mismatched fields
should be documented as part of the fix.

[kyrieb-ekat]

@etosphere encountered this behavior, so any details you have please add if I missed anything.

When I was in there earlier, I didn't actually verify the CSV column mismatch; when I tried to run chant import locally, it failed earlier with urlopen(None) because my fixture manuscript had no csv_export_url set, so I never got as far as the parsing step. I drafted the issue based on Eto's description and the field names in chant_importer.py but I haven't compared them against a real CantusDB CSV export.

@liampond do you have a recent csv from your cantorales work we could check headers against?

[liampond]

Just emailed you!