feat(api-go): migrate workspace themes

[houko]

Summary

• migrate the five Workspace Theme list/create/retrieve/patch/delete routes to Gin/GORM
• preserve Django admin/member access and guest denial
• preserve serializer defaults, JSON colors, name length/uniqueness, writable audit fields, and response/error shapes
• preserve soft deletion and the Django-compatible soft_delete_related_objects Celery task
• cut over only the Workspace Theme collection and UUID detail paths; all other Workspace submodules remain on Django

Verification

• cd apps/api-go && go test -count=1 ./...
• cd apps/api-go && go test -race -count=1 ./...
• cd apps/api-go && go vet ./...
• git diff --check
• Caddy 2.11.4 configuration validation
• rollback-only GORM create/update/delete coverage against PostgreSQL created from authoritative Django migrations
• positive/negative proxy-boundary tests

Migration sequence

This PR targets main directly. No later migration PR will be opened until this PR is merged.