Plan for dropping unpreserved nma_pk_* columns after transfers are permanently complete

[kbighorse]

Once legacy transfers from NM_Aquifer are permanently complete, the following columns are only used for transfer/audit and can be dropped after the prerequisites below. No application business logic depends on them.

Columns that can be dropped (unpreserved elsewhere):

• Observation.nma_pk_waterlevels
• Sample.nma_pk_waterlevels
• Thing.nma_pk_welldata (after removing from OGC views)
• Thing.nma_pk_wellscreens
• Contact.nma_pk_owners (after removing from search vector)
• Contact.nma_pk_waterlevels
• Sensor.nma_pk_equipment

Prerequisites before dropping:

1. PyGeoAPI: Update OGC thing views (e.g. in migration d5e6f7a8b9c0 or a new migration) to stop selecting thing.nma_pk_welldata, then drop the column.
2. Search: Update the contact search vector definition so it no longer includes nma_pk_owners (see e4f7a9c0b2d3_add_search_vector_triggers.py and Contact model), then drop the column.

Scope: Add a short doc or ADR that records this plan and the prerequisites; implement the drop only when the team has decided transfers are permanently complete.