Bug: UpsertBatch's ON CONFLICT clobbers external_ids on OSM re-ingest

[koinsaari]

Bug

internal/place/repository.go UpsertBatch's ON CONFLICT DO UPDATE column list includes external_ids:

DoUpdates: clause.AssignmentColumns([]string{
    "name", "lat", "lng", "category", "rank", "tags", "external_ids", "status", "updated_at",
}),

internal/sources/osm/transformer.go TransformNode only ever populates ExternalIDs with the OSM entry:

ExternalIDs: models.ExternalIDs{
    "osm": models.ExternalRef{ID: fmt.Sprintf("node/%d", osmID), Confidence: 1.0},
},

Any subsequent OSM full-import or diff-sync run over an already-ingested node overwrites the entire external_ids JSONB column with just {"osm": ...}, silently destroying any Wheelmap (or other future external source) ref that AttachExternalRef's jsonb_set had previously attached to that place.

This contradicts the documented design (AGENTS.md: "AttachExternalRef is the only path that writes it") and defeats the point of using jsonb_set for surgical per-source updates in the first place.

Why it wasn't caught

No existing test re-runs UpsertBatch on a place that already has a non-OSM external_ids entry attached. TestAttachExternalRef_ExistingDifferentKey / TestAttachExternalRef_ExistingSameKey (internal/place/repository_integration_test.go) seed once, attach once, check once — none re-ingest afterward.

Fix

Drop external_ids from UpsertBatch's DoUpdates list. OSM's own entry only needs to be written once, at insert time (Create already sets it via the row's own ExternalIDs field); on conflict it should be left untouched, exactly like the code comment already claims.

Suggested test

Extend the existing AttachExternalRef integration tests (or add a new one) to: seed via UpsertBatch, attach a wheelmap ref, re-run UpsertBatch with the same (osm_id, osm_type), then assert the wheelmap entry survives.