Partial covering index on versions for latest dependent_packages join

[andrew]

Adds index_versions_on_id_where_latest_covering on versions (id) INCLUDE (package_id) WHERE latest = true, built concurrently with statement_timeout cleared.

The latest_dependent_packages / latest_dependent_package_kinds queries currently nested-loop probe versions_pkey and heap-fetch each row to read package_id and filter latest. With ~14M latest versions out of 163M total this index lets that probe become an index-only scan against a ~500MB partial index, and rows surviving into the HashAggregate/Sort drop from one-per-dependency-row to one-per-dependent-package.

Follows on from the API default flips so the default dependent_packages and dependent_package_kinds paths hit the latest = true branch and can use this.

Doesn't help the ?latest=false opt-out, update_dependent_packages_count, or the dependencies-side scan for npm-scale targets.