FAC - Feature: Restore EIN subcode to /general output

[stucka]

Is your feature request related to a problem? Please describe.

The EINSUBCODE system was not used consistently, but was used in a thin majority of cases where there were EIN overlaps in the historical system. This helped identify directly comparable companies. While it wasn't a truly unique index, it eliminated about half of the ambiguity that remains when other old-system-specific fields are excluded.

This data item is not present in the current /general API output, but does appear to be worked with, e.g.:
backend/data_distro/mappings/upload_mapping.py: "EINSUBCODE": ["Auditee", "ein_subcode"]

Describe the solution you'd like

Consider bringing back the EIN subcode to better track historical data across years.

Describe alternatives you've considered

No response

Additional context

The CSV has examples of EIN and fiscal year end overlap from historical data. There are roughly 24,000 entries that overlap in that way, and roughly 13,000 have EIN subcodes that could distinguish them.

old-csv-index-attempt1.csv

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