config-as-code: migrate label/moderation/review-evasion settings off the DB

[JSONbored]

Part of #6440.

Fields: `gittensorLabel`, `blacklistLabel`, `createMissingLabel`, `typeLabelsEnabled`, `typeLabelsJson` (`typeLabels`), `linkedIssueLabelPropagationJson` (`linkedIssueLabelPropagation`), `contributorBlacklistJson` (`contributorBlacklist`), `moderationGateMode`, `moderationRulesJson` (`moderationRules`), `moderationWarningLabel`, `moderationBannedLabel`, `reviewEvasionProtection`, `reviewEvasionLabel`, `reviewEvasionComment`, `mergeTrainMode`.

All already parse correctly from `.loopover.yml`'s `settings:` block (confirmed via audit, including the sparse-merge composite fields `typeLabels`/`linkedIssueLabelPropagation` which must not let a partial override reset sibling keys). DB-removal half: backfill, stop reading as merge base, drop columns, remove from dashboard write path.

[linear-code]

JSO-4439

[JSONbored]

Confirmed ready as-is — no embedded open question here, unlike #6444/#6446. All 15 fields already parse
correctly from .loopover.yml's settings: block, including the sparse-merge composite fields
(typeLabels / linkedIssueLabelPropagation) which the audit specifically checked don't let a partial
override reset sibling keys. This is a DB-removal-only mechanical migration with clear precedent
(backfill → stop reading as a merge base → drop the DB columns via migration → remove from the dashboard
write path) and no business decision involved. Unlocking for contributors.