Add soft-delete for service variants

[jackfreem]

Problem

Backfila supports service variants (e.g. customermappings/playpen-jackf, customermappings/playpen-<ldap>) but there was no way to delete them once created. Stale playpen variants accumulate over time with no way to clean them up through the UI or API.

Solution

Adds soft-delete support for service variants. A deleted variant is hidden from all listings and detail views, and can be re-registered fresh if needed.

Changes

Database

• Adds deleted_at TIMESTAMP(3) NULL column to the services table (v024__backfila.sql)
• Adds softDelete(clock) method to DbService
• Adds ServiceQuery.notDeleted() constraint using IS_NULL on deleted_at

Backend

• New DeleteServiceVariantAction: POST /services/{service}/variants/{variant}/delete
  • Requires users capability
  • Guards against deleting a variant that has running backfills
  • Returns 400 if the variant doesn't exist or is already deleted
• Filters deleted variants from GetServicesAction, GetServiceVariantsAction, and GetServiceDetailsAction
• Updates ConfigureServiceAction to skip deleted variants in its lookup, so re-registering a deleted variant creates a fresh row

UI

• Adds a "Danger Zone" section to the service info page (/services/{service}/{variant}/info) with a "Delete Variant" button and a confirmation dialog

Design decisions

• Soft delete only — preserves historical backfill run data that references the service_id FK, consistent with how backfill run deletion works
• Re-registration creates a fresh row — deleted variants are excluded from the ConfigureServiceAction upsert lookup, so calling configure_service after deletion starts clean
• Block deletion if running backfills exist — prevents orphaning active work

Testing

Added DeleteServiceVariantActionTest covering:

• Happy path deletion
• Deleted variant not appearing in services/variants listings
• 400 on non-existent or already-deleted variant
• 400 when running backfills exist
• Re-registration after deletion creates a fresh row

Once the migration is run, I'll be able to test the remaining parts of this.