Implements new approach for prebuild invalidation as discussed in issue#17917.Instead of directly deleting workspaces, this sets a timestamp on presets,and the reconciler automatically marks prebuilds as expired and creates new ones.Implementation:- Add last_invalidated_at column to template_version_presets table- API endpoint: POST /templates/{template}/prebuilds/invalidate- Updates last_invalidated_at = NOW() for all active template version presets- filterExpiredWorkspaces checks if prebuild.created_at < preset.last_invalidated_at- Reconciler automatically deletes expired prebuilds and spawns fresh onesChanges:- Migration 000394: Add last_invalidated_at column- Query: UpdatePresetsLastInvalidatedAt sets timestamp on active presets- API: Returns count and list of invalidated preset names- Logic: filterExpiredWorkspaces prioritizes last_invalidated_at over TTL- SDK: InvalidateTemplatePrebuilds client method with structured responseBenefits:- Non-blocking: API doesn't wait for reconciler- Async: Reconciler handles actual deletion- Safe: template_version_presets is no longer write-onceUpdates#17917

- Use uuid.NullUUID{UUID: template.ID, Valid: true}- Use sql.NullTime{Time: dbtime.Now(), Valid: true}

This reverts commitf8a06c6.

Remove redundant subquery - template_version_id IN (...) AND template_version_id = (...)is unnecessary. Just check template_version_id = active_version_id directly.