Rulebase UI API2 read fallback removal

ENG-2554  removes the dead Rails read branches behind the globally enabled API2 settings rollouts. The affected queries now call API2 directly; response shapes, query keys, cache behavior and the surrounding feature boundaries are unchanged.

Ownership boundary

This package removes 20 Rails-client importer files from the source/test census (311 to 291), including 19 runtime importers (300 to 281), and deletes all 24 references to the four unconditional settings-read helpers. It covers reads for assignment queues, complaint categories, conversation review notes, QA evaluation requests, organization tags, digest emails, evaluation settings, knowledge-base sources and folders, macros, pinned conversations, redaction rules, scorecard alert rules, SLA policies, team sampling and work schedules.

Digest-email and redaction-rule mutations still use Rails. Other UI Rails reads without an API2 contract are outside this package and remain visible in the client-import census. The removed rollout helpers are not rollback switches: they returned true in every environment, so rollback is the previous UI bundle.

Verification

Before merge, run Rulebase UI typecheck and lint plus focused tests for Slack channel settings, conversation tags and team weekly sampling. Confirm the helper names have no references and recount exact Rails-client importer files.

After the normal release deploys the UI, first confirm Sentry dashboard 5423172 is receiving the first_party_client:rulebase-ui tag. Then observe the removed endpoint families over a complete production window and investigate any Rails request before taking retirement credit. Admin remains tracked but outside the UI milestone. This package requires no database migration, feature-flag change, manual deploy or production mutation.