Role merge and rename #2476

The taxonomy-side twin of person merge (which already exists).

The situation

A scrape hits a title with no matching alias. Under the new ingest behaviour it lands as a provisional role term rather than being dropped. Later, a human recognises it as an existing canonical role under a different name — and needs to merge it.

This is structurally the same operation as merging two people, and it needs the same care: the two identities collapse into one and their histories must combine, not be lost.

Operations to support

Merge role B into role A

• Append a merge event; do not rewrite the observation log (it is append-only).
• Project history through the merge: first_seen_at for the merged role = the earlier of A's and B's.
• Record the alias. role_aliases.source already has a 'learned' value in migration 095 — this is exactly what it is for. Once learned, the next scrape normalizes that title automatically without a human.

Rename role A

• Display-string change only. Identity (role_terms.id) is preserved, so no history resets.
• This only works if everything downstream references the term id and never the display string — enforced by the taxonomy issue.

Open question

Does a locality override create a distinct role identity?

role_terms is unique on (value, jurisdiction_ocdid), so an override is a new row. The proposal is that it must not be a new identity — otherwise every locality override forks history for the people it touches. Worth confirming the semantics we want before building the merge UI, because merge and override interact: merging a provisional local role into a state canonical is the common case.

Depends on

• Taxonomy re-rooted, roles keyed on role_terms.id
• Observation log exists and is merge-projectable
• Ingest change (provisional roles must exist to be merged)

Related

Person merge is already implemented — reuse its patterns and, where possible, its UI shape. See the review/merge model in civicpatch.org/src/frontend/components/review/merge-model.ts.

Part of the temporal-roles work — see the tracking issue for ordering.