fix(etl): don't delete rows from an ETL migration

[rickyrombo]

Follow-up to #425. Pairs with AudiusProject/api#1010.

Problem

0035 added the users current-row arbiter and, because violations already existed, deleted the duplicates first. ETL migrations run automatically at indexer start (SkipMigrations is left false by api), so that made go get github.com/OpenAudio/go-openaudio/pkg/etl@… a thing that silently removes rows from a consumer's production database.

Whatever the merits of the repair, a dependency bump should not be able to do that.

Split

• index stays here — it's the ETL's invariant, and it belongs beside the four 0030 added.
• repair moves to the owning application — api ddl 0237_users_one_current_row_backfill.

Ordering is guaranteed, not hoped for

The index cannot be created while duplicates exist, so the backfill must run first. api's deployment guarantees it: bridge migrate runs as a pre-roll Job that every serving Deployment DependsOn, with runMigrations=false on the serving pods. The ddl migrations therefore complete before an indexer carrying this migration can start — and a failed migration blocks the rollout instead of half-applying.

Both orders verified against a fixture:

index first          -> ERROR: could not create unique index "users_current_uniq_idx"
                        DETAIL: Key (user_id)=(98311147) is duplicated.
backfill then index  -> both apply; violations 0; indisvalid = t
re-running either    -> no-op

Failing loudly in the wrong order is the intended behaviour: it means the owning application has not run its backfill. Databases built solely by this indexer cannot contain duplicates, so they are unaffected.

Note on v1.6.3

pkg/etl/v1.6.3 shipped the delete-and-index version of 0035. Nothing has applied it — :stable is still v1.8.2 and api has not merged its bump — so no database has run it. api#1009 (which pins v1.6.3) is safe either way: with api#1010 merged, the ddl backfill runs first in the pre-roll Job and 0035's delete finds nothing to do.

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